Question

What does f(4)= -3, f(0)= -2 mean?

Answer 1

A function is

f(x) = result

This means that given a value of x you will get a certain result

So what your equation:
f(4) = -3
means is that the function given the x with the value 4 we get a result of

And the other equation:
f(0) = 2
means that the function given the x with the value 0 we get a result of 0

Answer 2

obviously there is a function called 'f'... this has a pronumeral in it... f(4)=-3 means that when you replace the pronumeral in the function with 4, it equals -3... f(0)=-2 means that when you replace the pronumeral with 0 it equals -2... hope that helps