HELP TO UNDERSTAND CODE!!!

PLEASE HELP ME TO UNDERSTAND CODE, I tried to make it but does not work. The only thing I do not understand where I need to connect pin 10 (int pin p = 10;). what is pin p, where I connect it. thanks

below is the link of the video on Youtube

/*
Stop-Watch
Push button re-start the timer
*/

int a = 5;
int b = 7;
int c = 9;
int d = 11;
int e = 12;
int f = 6;
int g = 8;
int p = 10;

int d4 = 4;
int d3 = 3;
int d2 = 2;
int d1 = 1;

int startStopReset = 13; 

long n = 30000; //start time: 300000 = 30s seconds
int x = 100;
int del = 55; 
 
void setup()
{
  pinMode(d1, OUTPUT);
  pinMode(d2, OUTPUT);
  pinMode(d3, OUTPUT);
  pinMode(d4, OUTPUT);
  pinMode(a, OUTPUT);
  pinMode(b, OUTPUT);
  pinMode(c, OUTPUT);
  pinMode(d, OUTPUT);
  pinMode(e, OUTPUT);
  pinMode(f, OUTPUT);
  pinMode(g, OUTPUT);
  pinMode(p, OUTPUT);
  pinMode(startStopReset, INPUT); 
  digitalWrite(startStopReset, HIGH); 
}
 
void loop()
{
  clearLEDs();
  pickDigit(1);
  pickNumber((n/x/1000)%10);
  delayMicroseconds(del);
 
  clearLEDs();
  pickDigit(2);
  pickNumber((n/x/100)%10);
  delayMicroseconds(del);
 
  clearLEDs();
  pickDigit(3);
  dispDec(3);
  pickNumber((n/x/10)%10);
  delayMicroseconds(del);
 
  clearLEDs();
  pickDigit(4);
  pickNumber(n/x%10);
  delayMicroseconds(del);
 
  n--; //'n++' for stopwatch
 
  if (digitalRead(13) == LOW)
  {
    n = 20000; //re-start time: 200000 = 20s seconds
  }
}
 
void pickDigit(int x) 
{
  digitalWrite(d1, HIGH);
  digitalWrite(d2, HIGH);
  digitalWrite(d3, HIGH);
  digitalWrite(d4, HIGH);
 
  switch(x)
  {
  case 1: 
    digitalWrite(d1, LOW); 
    break;
  case 2: 
    digitalWrite(d2, LOW); 
    break;
  case 3: 
    digitalWrite(d3, LOW);
    digitalWrite(p, HIGH); //new 
    break;
  default: 
    digitalWrite(d4, LOW); 
    break;
  }
}
 
void pickNumber(int x)
{
  switch(x)
  {
  default: 
    zero(); 
    break;
  case 1: 
    one(); 
    break;
  case 2: 
    two(); 
    break;
  case 3: 
    three(); 
    break;
  case 4: 
    four(); 
    break;
  case 5: 
    five(); 
    break;
  case 6: 
    six(); 
    break;
  case 7: 
    seven(); 
    break;
  case 8: 
    eight(); 
    break;
  case 9: 
    nine(); 
    break;
  }
}
 
void dispDec(int x)
{
  digitalWrite(p, LOW);
}
 
void clearLEDs()
{
  digitalWrite(a, LOW);
  digitalWrite(b, LOW);
  digitalWrite(c, LOW);
  digitalWrite(d, LOW);
  digitalWrite(e, LOW);
  digitalWrite(f, LOW);
  digitalWrite(g, LOW);
  digitalWrite(p, LOW);
}
 
void zero()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, HIGH);
  digitalWrite(f, HIGH);
  digitalWrite(g, LOW);
}
 
void one()
{
  digitalWrite(a, LOW);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, LOW);
  digitalWrite(e, LOW);
  digitalWrite(f, LOW);
  digitalWrite(g, LOW);
}
 
void two()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, LOW);
  digitalWrite(d, HIGH);
  digitalWrite(e, HIGH);
  digitalWrite(f, LOW);
  digitalWrite(g, HIGH);
}
 
void three()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, LOW);
  digitalWrite(f, LOW);
  digitalWrite(g, HIGH);
}
 
void four()
{
  digitalWrite(a, LOW);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, LOW);
  digitalWrite(e, LOW);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}
 
void five()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, LOW);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, LOW);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}
 
void six()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, LOW);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, HIGH);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}
 
void seven()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, LOW);
  digitalWrite(e, LOW);
  digitalWrite(f, LOW);
  digitalWrite(g, LOW);
}
 
void eight()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, HIGH);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}
 
void nine()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, LOW);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}

https://www.youtube.com/watch?v=HfmEA28xg7E

Probably be the decimal dot on (one of) the 7 segment display(s).

But I doubt that will be the cause of a non-working code. I don't do youtubes so can't really help further.

Dear friend,
from your posts, public and private , I gather there are some issues with communicating your thoughts.

May I suggest you comment your code instead of just "cut and paste" it?
Your last code obviously writes into a device ( 7 segment LED) where the individual segments are customary labeled a thru g , plus decimal point - p. You can get more description on the device from datasheets. .

You could also add simple comment to your code

// seven segment LED input pins
int a = 5;
int b =...

int p = 10; // decimal point

It will make it more "user friendly" and explanatory about what hardware you are using.

And NO , nobody should expect queens English comments.

Comments are part of coding and not to impress with your knowledge of English.

Good luck.

Jim

Rather than writing dumb comments like int p = 10;    // decimal point , if you give your variables (which really ought to be constants, and "byte"s, not "int"s) sensible names like "SEG_A", "SEG_B"... "SEG_DP", you don't need the comments.

Kudos, however, to the OP for using code tags correctly. If only more well-established members could be persuaded that it's a good idea.