A crossfader is a slider on a dj mixer that performs smooth transitions between an audio input on one channel and an audio input on another channel. If you have two songs playing into the two seperate channels, and the crossfader is in the middle, then both will be playing at the same level, modified only by the volume sliders. If you move the crossfader towards one channel, it will become more prominent as the other dies out, and vice versa. Some mixers include a hamster switch which will reverse the crossfader with one touch.