Step 4: Auxiliary outputs (Auxes)
The most common use for auxes is stage monitors. Lets say that we have Bob on stage singing, and Joe playing guitar, and each has his own monitor. Lets also say we have Bob's voice plugged into channel 1, and the guitar in 2, and Bob's monitor in aux 1 and Joe's in aux 2. So, if Bob wanted more guitar in his monitor, we would go to channel 2 (where Joe's guitar is plugged in) and turn up the Aux 1 knob: this tells the board to put more of the signal in channel 2 into the device plugged into aux 1.
Two other common uses for auxes are reverbs (device that simulate reverberation effects) and subwoofers (loudspeakers designed to reproduce lower frequencies than normal speakers can play).