The main function is the entry point for your application. However, this isn't called directly by the operating system because C++ will perform initialization before main is called. This includes constructing the Standard Library global objects (cin, cout, cerr, clog, and the wide character versions) and there is a whole host of initialization that is performed for the C Runtime Library that underpins C++ libraries. Further, there are the global and static objects that your code creates. When the main function returns, the destructors of global and static objects will have to be called and a clean-up performed on the C runtime.
There are several ways to stop a process deliberately. The simplest is to return from the main function, but this assumes that there is a simple route back to the main function from the point that your code wants to finish the process. Of course, process termination must be ordered and you should avoid writing code where it is normal to stop...