Every function must be named so that it can be used. The main function is unusual in that the name 'main' is never called in the program. Conceptually, it is the operating system that calls the main function. The other unusual item is that main is a C++ reserved word, unlike any other function name.
Some operating systems, most notably Windows 95/98/NT, disallow the use of main but require the function to be named WinMain, if the program is to use the Windows Graphical User Interface. Even in these, a console program will use main as the function name.
Some time later, the definition of functions other than main will be discussed. For the time, we will just copy the main function header as-is without modification.