what is function ?

Answer 1

Functions have a very important part in programming. They are used to avoid repeating code and are also a good way to group similar statements and ideas. Functions are meant to perform different tasks that the program might need. e.g. If I had my main() function taking an input of numbers and printing out the sum, I might want to have an add() function that I can call and then that function can return the sum rather than having my main() function do everything. Functions are a fundamental part of many languages so it's important to know how to use them properly.

More about Functions: https://en.wikipedia.org/wiki/Subroutine
Subroutine is just another word for function

Answer 2

function is group of statements that together perfor a task. every cop program has at last one function which is main().