Gain, a.k.a. Trim, is a volume control for each channel of a mixing console.  The Gain knob controls the volume being input into the channel, not the amount of volume output through the channel.  Different types of input devices, from mp3 players to guitars to vocal microphones all have different volume levels that are input into the mixing console.  The job of the Gain/Trim knob is to control those levels so the faders can best control the volume output.

Setting the gain for each channel can be done using channel meters within each channel, on the high-end mixers, or the volume meter located on the mixer via use of the Solo/PFL button.  The PFL/SOLO button acts as a controller over which channel is being monitored in the volume meter.
 

Set The Gain:

1. Select a channel for setting a level, let's say a vocal microphone in channel #4. 
2. Have the singer sing a verse or two of a song they will be singing that day - and sing at their normal level. 
3. With the PFL button pressed and the Gain knob set at zero, start turning the Gain knob up.  The fader should be down at this point. 
4. Stop turning the Gain knob when the volume meter is showing 2-3 green bars. 
5. Tell the singer to go up in volume. 
6. Turn the knob back if the meter goes into the red. 
7. Expect it to stay green with an occasional yellow. 
8. You don't want to be on the edge of green/yellow as when you add in other channels, every 3 channels increase the overall volume level.

Now, when you move the fader up to the 0 indicator, you will have the best control over the vocal volume. 

Using this method, it's possible to set all fader's to the 0 mark and have equal volume levels for all channels.  That doesn't mean the mix is finished, it just means the volume levels are in a good range.

[Note: I have seen instances where the fader had to be set up to 0 before the LED's would show up in step 3.]