Question

What is a function?

Answer 1

Depends who's asking, there are 2 valid definitions.
I suppose it also depends what level you are at...?

Answer 2

I guess you are not there right now so we will go with a simple version.
Think of it as a machine which takes inputs at one end and produces outputs at the other end in such a way that each input is associated with a particular output.
Inputs, outputs, association -> function