A function is a special relationship between values: Each of its input values gives back exactly one output value.
It is often written as "f(x)" where x is the value you give it.
f(x) = x/2 ("f of x is x divided by 2") is a function, because for every value of "x" you get another value "x/2", so:
* f(2) = 1
* f(16) = 8
* f(-10) = -5