This line is necessary because the return type of main is int (see above). We'll talk more about functions and return types later, but for now understand that because the function's return type is int, the function must return an int (integer). To return 0 (which is an integer, we simply write return 0;.
A simple program
return
//include this file for cout
#include <iostream.h>
int main()
{
//print out the text string,
//"Hello, World!"
cout << "Hello, World!"
  << endl;
return 0;
}