explanation for a few lines of code...

Folks, Newbie looking to learn. Current project is a clock. Working on the display (4 digit, 7 segment) SH5461AS currently. Then I'll move over to the RTC. I got ahead of myself on this project I think. Below are a few lines of code that I'd like to understand better.

// New to Arduino
// Trying to learn with little projects
// This based on some code I found and a schematic.  Eventually I'd like to build a simple clock.
// I believe this is a multiplexed 4 digit seven segment display,  There are 12 connections under the module.  6 on top and 6 on bottom.

#include "SevSeg.h"                               // Library for 4 digit, 7 segment display.

SevSeg sevseg;                                    // what does this mean?  What does it do?

void setup(){                                     // setup section - do these once
  byte numDigits = 4;                             // Set variable numDigits = 4, why use byte?  what not int for example?
  byte digitPins[] = {10, 11, 12, 13};            // Set array for Arduino as digitPins = {10, 11, 12, 13}, why use byte?  what not int for example?
  byte segmentPins[] = {9, 2, 3, 5, 6, 8, 7, 4};  // Set array for Arduino as segmentPins = (numbers), why use byte?  what not int for example?
  bool resistorsOnSegments = true;                // What is bool? Is this the same as putting "true" in the line "sevseg.begin..."  Can either be true or false, correct?
//  bool updateWithDelaysIn = true;                 // What is this?  Doesn't see to be needed.  Commented out.
  byte hardwareConfig = COMMON_CATHODE;           // Set variable hardwareConfig = COMMON_CATHODE.  Other options would be COMMON_ANODE, right?  Didn't want to test anything else for fear of burning something out.
  sevseg.begin(hardwareConfig, numDigits, digitPins, segmentPins, resistorsOnSegments);   // I guess this sets up the display.
  sevseg.setBrightness(200);                      // -100 to 0 and quite dim, 100 to 200 brighter but not much diff between 100 and 200
}

void loop() {                                     // loop section - do these over and over
  sevseg.setNumber(1234,2);                       // Send the numbers to the display.  Set dot location with second number.
  sevseg.refreshDisplay();                        // Refresh the display
}
#include "SevSeg.h"                               // Library for 4 digit, 7 segment display.

SevSeg sevseg;                                    // what does this mean?  What does it do?

There is a library that has a class. The class needs be instantiated so the class members can be accessed. The declaration SEVSeg sevseg is setting the class as an object you named sevseg.

What is does can be seen by looking at the class API which can be found at GitHub - DeanIsMe/SevSeg: Seven segment display controller library for Arduino.

This library turns your Arduino into a seven segment display controller. Use it to easily display numbers on your seven segment display without any additional controllers.

It supports common cathode and common anode displays, and the use of switching transistors. Numbers can be displayed in decimal or hexadecimal representation, with decimal places. Characters can be displayed (as accurately as possible). It also supports multiple displays, of varying dimensions. Shift registers and similar devices are NOT supported.

  bool resistorsOnSegments = true;                // What is bool? Is this the same as putting "true" in the line "sevseg.begin..."  Can either be true or false, correct?
//  bool updateWithDelaysIn = true;                 // What is this?  Doesn't see to be needed.  Commented out.
  byte hardwareConfig = COMMON_CATHODE;           // Set variable hardwareConfig = COMMON_CATHODE.  Other options would be COMMON_ANODE, right?  Didn't want to test anything else for fear of burning something out.
  sevseg.begin(hardwareConfig, numDigits, digitPins, segmentPins, resistorsOnSegments);   // I guess this sets up the display.

Sometimes their are multiple ways of doing the thing. Sometimes you may want to change a setting without recreating the class object, individuals property settings allow on the fly changes. You can look at the h file to see what properties / methods are available SevSeg/SevSeg.h at master · DeanIsMe/SevSeg · GitHub.

Good questions.

C++ classes and Objects C++ Classes and Objects

Capitalization is important.

SevSeg sevseg;   // Declares variable sevseg to be able to hold a SevSeg object.

// The SevSeg object is probably defined in the file SevSeg.h
// Microcontrollers tend to be short on memory.  On an Arduino, an int takes two bytes so is twice as much space
// as a variable declared byte... but an int ranges from -32768 to +32767 and 
// a byte is more limited AND unsigned, ranging from 0 to +255.
// See for example https://www.arduino.cc/reference/en/language/variables/data-types/byte/

You may want to look at byte - Arduino Reference
You also need to learn C++.