Can some explain me the diffrence between "return 0" and "return 1." or "return 2"?

Answer 1

Whatever you return from main typically gets fed back to whatever launched the program. A return value of zero indicates the program ran successfully, whereas nonzero indicates a failure of some kind.

For stand-alone programs it doesn't really matter all that much, as this code is often ignored. But for launching programs inside batch scripts, where multiple programs need to run in succession... an error code could halt the entire script.