Post-fader means that something is active after the fader. This is most commonly used when talking about effects, plugins, sends and inserts. As the fader controls the volume of the channel or track, putting something before or after the fader can have drastic effects on the volume of said thing. A common use for putting something post-fader is when you apply effects to a track or channel. You want the effects to be applied in a relative amount to how loud the track is playing, so that it always sounds like it has the same amount of effects on it, regardless of the sound level of the track. By putting the effect post-fader, its' effect will be relative to the overall sound level of the track.