In music, a patch is a sound, or the set of preset parameters that make up the sound, on a synthesizer or keyboard instrument (or an effects device with patch memory). For example, my Korg DW8000 had a halfway-decent Hammond organ patch.
The term itself derives from the days of modular synthesizers, when the individual modules (oscillators, filters, amplifiers and so on) had to be connected with patch cords in order to produce sounds.