What does Pre Fade Listen mean and do?

Pre-Fade Listen or PFL on a mixer / recorder allows you to listen to the sound source signal before it goes through the fader. This is useful to check ISO tracks are clean when monitoring a mix track where for instance 2 people are talking and one hits their radio mic but you didn't see who and on your mix track it sounds like it might have been the person talking because the signals are mixed together. Then you replay a PFL to separate the sources and see if they are clean.