Please explain this C program for me?

After your includes you have what is called a function prototype. This says to the compiler "this is a user defined function". Then in main() you can call the function just like you would printf() or any other standard function. At the end of main we describe what the function does. This is called the function body. Every time you call the function, it runs the code inside the body, then returns program flow to back to main() where it left off.