what does the code '' return 0'' do in c++?

Answer 1

Common misconception : Not in C++, but in main().
In main(), where main() returns an int datatype, return 0 is an exit code used to tell your compiler to stop processing the main function. You can return some other value but werid results may sometimes occur.

Answer 2

You can write "void main(){}" instead

Answer 3

The integer returned by main in C and C++ is considered the return status of the application. In most OSes 0 is considered a successful return and no problems happened.