The inverse function of f(x)=2x is g(x)=x/2. If you apply the function f to the number 7 you get 14. If you now apply the function g to the number 14, you get 7 back. That means that g undoes what f did. In other words, g is the inverse function of f (and viceversa).
In this video I explain that in order for a function to have an inverse function, it must be one-to-one and the I go on to show you how to calculate the inverse function of a one-to-one function.