Could someone tell me what this command is doing ?

t1.execute(&processPosition);

Thanks, and I know that this might be a weird question.
I think that the &processPosition is the address of processPosition.
If that is correct I dont know what t1.execute is supposed to be,
If it were simply t1(&processPosition); I think that it is calling a function titled t1 and passing it the address of
processPosition.

But then again I could have it all wrong.

t1 is a class object that was declared somewhere in the code you didn’t post. execute() is a method of that class. Neither fact has any bearing on the parameters that get passed.

‘t1(&processPosition)’ is an error because it tries to call a class object as if it were a function.

Take a look at the rest of the code to determine what "t1" refers to.

We can't, because you forgot to post the code.

t1 is an object created from a "used defined variable" which has been created using class keyword. The hierarchical chain could be found from the Library which the OP has not posted.

execute() is a method that has been applied on the object named t1.

GolamMostafa:
t1 is an object created from a “used defined variable” which has been created using class keyword.

Or the struct keyword.

TheMemberFormerlyKnownAsAWOL:
Or the struct keyword.

Yes!

TheMemberFormerlyKnownAsAWOL:
Or the struct keyword.

Sure, but "statistically" it is much more likely to be a class. Oh, the joy of answering an incompletely specified question...

lol, seems a common problem

Folks, I apologize for my original posting of only a snippet of the code. I will try and post more meaningful stuff here which will be where “t1” is referenced using a find command across all coding.

Hmmm, I am trying to post the main sketch but it exceeds the 9000 character limit.

Anyhow, here are all the references to t1 along with timer.cpp and timer.h

t1 find command and all t1s are in the main sketch.

/Non-blocking Timer object
Timer t1(100);<<<<<<<<<<<<<<<<<<<<<<1st reference to t1

t1.reset(100);<<<<<<<<<<<<<<<<<<<about 6 of these all in the main sketch

t1.execute(&processPosition); <<<<<<<<<<<<<<<<<<<,the stmt that I posted earlier

timer.cpp

//timer.cpp - Timer library.
//Copyright (c) 2015-2018 Julie VK3FOWL and Joe VK3YSP
//For more information please visit http://www.sarcnet.org
//Released under the GNU General Public License.
//Provides a non-blocking millisecond timer object that can be used to replace the delay() function

#include "timer.h"

Timer::Timer() {
  //Use this constructor if you don't need to set a period
  reset();
}

Timer::Timer(long period) {
  //Use this constructor if you need to set a period
  _period = period;
  _last = 0;
  reset();
}

void Timer::reset() {
  //Resets the start time to the current time since power up
  _start = millis();
  _last = 0;
}

void Timer::reset(long period) {
  //Resets, as above and sets the period
  _period = period;
  reset();
}

bool Timer::timeout() {
  //Returns true if the period has elapsed
  return (elapsed() >= _period);
}

long Timer::elapsed() {
  //Returns the number of milliseconds since reset
  return millis() - _start;
}

long Timer::periods() {
  //Returns the number of periods since reset
  return long(elapsed() / _period);
}

bool Timer::toggle() {
  //Return value toggles every period
  return (periods() % 2); //Returns true for odd and false for even using the mod function
}

bool Timer::tick() {
  //Returns true if the period has elapsed since the last call
  long p = periods();
  if (p > _last) {
    _last = p;
    return true;
  } else {
    return false;
  }
}

void Timer::execute(void (*f)()){
  //Executes function f once every period
  if (tick()){
    (*f)();
  }
}

timer.h

//timer.h - Timer library.
//Copyright (c) 2015-2018 Julie VK3FOWL and Joe VK3YSP
//For more information please visit http://www.sarcnet.org
//Released under the GNU General Public License.
//Provides a non-blocking millisecond timer object that can be used to replace the delay() function

#ifndef TIMER_H
#define TIMER_H

#include <Arduino.h>

class Timer {
  public:
    Timer();                  //Use this constructor if you don't need to set a period
    Timer(long period);       //Use this constructor if you need to set a period
    void reset();             //Resets the start time to the current time since power up
    void reset(long period);  //Resets, as above, and sets the period
    bool timeout();           //Returns true if the period has elapsed
    long elapsed();           //Returns the number of milliseconds since reset 
    long periods();           //Returns the number of periods since reset
    bool toggle();            //Returns a value that toggles every period
    bool tick();              //Returns true if the period has elapsed since the last call
    void execute(void (*f)());//Executes function f once every period
  private:
    long _period;             //Period in milliseconds
    long _start;              //Start time in milliseconds since power up
    long _last;               //Last value of periods
};
#endif

t1 is an instance of the class Timer

here is the execute method of that class:

void Timer::execute(void (*f)()){
  //Executes function f once every period
  if (tick()){
    (*f)();
  }

So every so often (defined by tick()), the function processPosition() gets called.

Now show us the declaration of ‘processPosition’. According to the timer.h and timer.cpp files, it should be a function that takes no arguments and returns no value. If so, the ‘&’ prepended to the name in the call to execute is superfluous. i.e. this would work just as well:

t1.execute(processPosition);

gfvalvo:
Now show us the declaration of 'processPosition'. According to the timer.h and timer.cpp files, it should be a function that takes no arguments and returns no value. If so, the '&' prepended to the name in the call to execute is superfluous. i.e. this would work just as well:

t1.execute(processPosition);

and here is processposition.

void processPosition() {
  //Perform the main operation of positioning the rotator under different modes
  //Read the accelerometer and magnetometer
  lsm.readGM();
  switch (mode) {
    case debugging:
      printDebug(); //Print the raw sensor data for debug purposes
      break;
    case calibrating:
      calibrate();  //Process calibration data
      break;
    case pausing:
      azMot.halt(); //Stop the AZ motor
      elMot.halt(); //Stop the EL motor
      break;
    default:
      lsm.getAzEl();  //Get the azimuth and elevation of the antenna                                                              //Get the antenna AZ and EL
      az = lsm.az;
      el = lsm.el;
      getWindup(&windup, &azWindup, &azOffset, &azLast, &elLast, az, elSet);      //Get the AZ windup angle and windup state
      if (mode == demonstrating) getAzElDemo(&azSet, &elSet, &azInc, &elInc);     //Set the AZ and EL automatically if in demo mode
      getAzElError(&azError, &elError, &windup, &azSet, elSet, az, el);           //Get the antenna pointing error
      if (mode == monitoring) printMon(az, el, azSet, elSet, azWindup, azError, elError); //Print the data if in monitor mode
  }
}

Yup. So, @evanmars provided the answer.

evanmars:
t1 is an instance of the class Timer

So every so often (defined by tick()), the function processPosition() gets called.

And is this the statement that defines?, or instances Timer ? If so, what is the (100) being passed to it and is t1 a shortened and alternate name for the instance Timer?

Timer t1(100);

No, Timer is the name of the class
An instance of the class is called and object, and one such instance is t1.

The value 100 will be explained in the source of the class.

Edited: bloody autocorrect

TheMemberFormerlyKnownAsAWOL:
An instance of the class is called and object, and one such instance is T1 t1.

The value 100 will be explained in the source of the class.

Right, from timer.h:

Timer(long period);       //Use this constructor if you need to set a period