Every C++ program starts executing at int main(). The compiler looks for this function, runs the code inside its curly braces, then stops. Without main, the compiler does not know where to begin.
The int before main means this function returns an integer number. The return 0; at the end sends the number zero back to the operating system, signaling the program finished successfully.
Non-zero numbers signal errors. You can only have one main function per program. If you write two, the compiler complains it does not know which one to run. Think of main as the door into your program.