Question

Why does f(-1)=0?

Answer 1

do you have f(x) function?

Answer 2

Is there more information to go with this? If you mean in a general sense, think of planar coordinates. Like, when you look at a graph and put points on it, they have an x coordinate and a y coordinate. F(x)=y is how that is usually interpreted which basically says "given a particular value of x, the function F will give (your y value) as its output. So, in this case, if you have a function, and set the value of x to -1, it will return 0 as your y coordinate.