This is not a class room where you will get the code explained.
Here, you can discuss your problem you are getting in your project or if you are stuck somewhere in your code. By this way experts here stop looking at your problem and you won't get your problem solved.
So, please ask only relavent questions.
By the way, the above code is calculating the factorial of 4.
Google for detailed information about Recursion.
Hope you will understand and won't repeat your mistakes again.