the auxilliaries are usually taps of the signal that run in parellel to your main path instead of in series with it the way fx inserts do.
They are often labelled as pre fader or post fader. Prefader sends do not take into account the fader level and therefore are good for things like headphone mixes. Post fader sends usually take into account your fader and pan control and as such they are good for sending to reverbs and other sorts of parallel fx