A preset, in the theatrical lighting sense, can be one of two thing.

Firstly, it could be the lighting state that the stage is in while the audience enter the auditorium. Called a preset, because it is how the lights are set, pre-show.

The other thing that a preset can be is a state on the lighting desk that can be controlled by a single fader, or flash button. Presets, in this context, are most useful when doing the plot. You would have different washes, for example, on different presets, so you wouldn't have to put 6 or so seperate channels up each time you wanted that state in a cue. Instead, you would just push the preset fader up to the level that you want the wash to be at.

If you're after any other theatre tech information, then do take a look at 'Everything you ever wanted to know about theatre tech, but were afraid to ask'