main() is a function. A function is a piece of code that does certain tasks with information they receive, and that gives out certain information.
Suppose you are a boss and you tell one of your employees to do something, and you give him some materials to work with (arguments). He will then go and do whatever process he needs and will come to you with the result (returned to you via the return statement).
So... a function in C has the following parts:
variable_returned indicates the type of variable that will be returned, say, an integer. Hence... in the function, if you return 3, for instance... the instance that called your function will receive value 3. Let's look at an example:
variable = returnsThree();
The value of variable will be 3.
Now, in int main(), the return statement helps the operating system to know whether the program did a normal exit or not. Suppose you interrupted it... then, you will be able to see that its exit code is something different to 0... in that way, the return statement helps you to know what kind of exit took place.
I hope I was clear enough :P And sorry if there's any syntax error in my previous code :P I haven't programmed in C for like 3 months