Unity gain is where the operating level of the equipment is achieved. On a preamp gain, this would entail providing enough gain from the preamp for the source going into the microphone is producing a either -10 or +4 level at the output of the mixer.
Unity on a fader or an aux send is simply the point of the fader or potentiometer where if there was a -10 or +4 level feeding to it, it would output that much also. So, sort of "no gain, no cut" like said earlier.
Usually you know you have achieved unity gain on a preamp by employing the PFL or Solo button on the channel, and looking at the meter that handles PFL or Solo functions. You would select the PFL (Pre Fader Level) for the preamp you are trying to set, the adjust the Trim or Gain knob of the preamp until the meter that handles PFL functions displays 0db.
Unity level on a fader will usually have a 0 mark for unity gain. On some console, all the way up is unity level.
On Aux sends, if all the way up is not unity level, then usually around 7 is.
You should thoroughly investigate how your particular console deals with gain structure. This is good stuff to know, because you really do want to provide unity gain levels at the output of any buss on the console.
Remember, if you are summing two things together, if they are both at unity gain, you will have a +6 db level at the output. Decreasing the two induvidual things by 3db apiece will of course make the combined level at unity at the output. If you have 4 sources combined, and all playing at once that are at unity gain, you will have a +12db gain at the output....etc.....
Confused yet?
Ed