Question

return 0 defines what

Answer 1

The return value of the main function is considered the "Exit Status" of the application.

Comments

In the main function, return is a way to complete a programs functionality successful or not.
For example.

#include <iostream>
using namespace std;

int main(){
    bool fileFound = false;
    //Find file then set the bool.
    if (!fileFound){
        cout<<"Error: File couldn't be loaded."
        return 1;
    }
    //continue the program
    return 0;
}

---

returning 1 means error, while returning 0 means success.