What does it mean and what is the purpose of 'Return 0'?

Question

I recently started some studies on C/C++ and in the vast majority of the initial examples I saw, I came across return 0 at the end of the code.

    #include<iostream>
    using namespace std;

    int main()
    {
        cout << "Hello World!";
        return 0;
    }

That one return 0 really returns zero? What is your purpose?

Sources

• cplusplus - Structure of a program
• Codingunit - First C++ program, hello world
• Wikiversity - C++Course/Helloworld Program
• University of Michigan - Hello world! Example Program (The C++ Programming Language)