Functions are the parts of your code that do something. Your functions are where you define the behaviour of your application.
You have already met some functions: count, conj, first, rest
Maths operators are functions too: * +, -, *, /
We can write our own functions too.
Arguments are values given to a function that could change the result of calling the function.
A Function always returns a value, even if that value is nil.
Always returning a value makes it easy for Clojure to use functions as an argument to another function.
- Reference: Basics of Function