The most basic idea behind it. You run a go route or basically run straight. However you fade or move towards the sidelines, but still in a forward direction. It is designed to get separation between the WR and CB with the QB having the ability to throw it on the outside shoulder of the WR so the worst that can happen is an incomplete.
The most important part by the WR on a fade route is to beat the jam with quick hands and make the CB believe he is going inside on a slant. This is where head fakes, hands and feet come important. You need separation on a fade route or else the CB will ride you.
The fade is most affective on man isolated coverage. Other wise, when a CB is giving you a cushion, that is what he is expecting and he is gonna be in position for it.