The main mix is the audio output that the audience will hear through the master speakers.
Technically the main mix is what's called a “bus” or a “signal path” to the master left and right output. This is a ‘stereo output” meaning that there are two different signals and essentially cables to run to two separate speakers.
It is important to start to visualize these “signal paths” with particular focus on the main mix bus, because you must know what signal you are sending to the audience and what signal you want to be only in your headphones.
When the volume faders are up and the cross fader is open, the signal from the channel will be sent through to the master mix, you can then use the volume fader to alter the level of the track to the master mix. You can also use the Cross fader to fade between songs and send the corresponding faded song to the master mix.
Mono or Stereo
The main mix can be either mono or stereo. Stereo will send the mix to left and right channels respectively and Mono summing will combine the left and right signal together and then send the combined signal through both speakers. A mono mix will allow slightly more volume, but the space perception of the panning of the music will be lost.
This also means certain effects will not be as effective when mono summed.A