Official comment
It could be a couple of things...
With the MASTER CUE enabled (arrow on the right), you'll be sending the master signal to the headphones.
With the channel cue enabled (either cue button in the middle, above the fader), you'll also be sending one or both channels (pre-fader) to the headphones.

Using the knob at the left, you can mix between the two sources (master cue / channel cue) in your headphones.
Try playing around with those functions to familiarize yourself with their operation.