What is a funtion in math terms?

Answer 1

A function defines a relationship that for every input, you get a unique output.

That's sounds a little complicated, so here's an example:

f(x)=2x

when x=1, f(x)=2

when x=2, f(x)=4

and so on...

You can see that whatever x is, f(x) will be different each time. If a "function" is not actually a function, then 2 different x's might give you the same f(x).

I hope you can understand what I just said.