A trapdoor function is a function that is easy to perform one way, but has a secret that is required to perform the inverse calculation efficiently.
A trapdoor function is a function that is easy to perform one way, but has a secret that is required to perform the inverse calculation efficiently.
That is, if $f$ is a trapdoor function, then $y = f(x)$ is easy to compute, but $x = f^{-1}(y)$ is hard to compute without some special knowledge $k$. Given $k$, then it is easy to compute $y = f^{-1}(x, k)$.
The related Wikipedia article also contains a straight-forward explanation.
