You may want to review function notation and one-to-one functions first.
The important thing about one-to-one functions is that they have inverses. The inverse of a function is the function that undoes it. For example, if the function is f(x) = −3x, then its inverse if f−1(x) = x / (−3), since dividing by −3 and multiplying by −3 undo each other. (f−1 is just a symbol for the inverse of f.) If f(x) = 2x + 1, then f−1(x) = (x − 1) / 2. Note that we have to switch the order; in f we first multiply by 2 and then add 1, so to undo it we have to first subtract the 1 and then divide by the 2.
There are some functions that are their own inverses. One example is f(x) = −x, since that undoes itself. Another example is f(x) = x, since then there's nothing to undo!