A simple way to think of Cue-Mix would be:
Picture yourself recording a 4 piece band (lets say drums, bass, guitar, and vocals) Let's say that they all wanted to record at the same time, without overdubbing each player individually (rare sometimes but does happen). Now, while they are recording, you put them in separate places so that you can isolate the sounds a bit, and they all have to wear headphones to hear each other. Cue mix lets you make 4-separate stereo submixes on the fly. So while they are recording, you can give the vocalist less guitar, and more bass, maybe give the guitar player mostly drums and vocals, the bass maybe wants only drums in his phones, and so on. Cue-mix allows you to give them each a mix while not affecting the levels that are being recorded to your computer.