When you play your organ, you can spend a lot of time picking out the different sounds to use for your song. You really don't want to use the same sounds for "Ave Maria" as you would for "Beer Barrel Polka"! Most organs, digital pianos and larger keyboards have presets to make this process much easier and help you choose appropriate settings.
The presets combine sounds along with the loudness or softness of each adjusted to sound good. A rhythm or musical style may or may not be part of the preset setup. On pipe organs they may be called “pistons,” on electronic organs, they may be called “presets” or “set-ups” or “registrations,” or “auto setup.” They may be set up in advance by the manufacturer and/or the player can make and save his own, but all accomplish the same goal - making you sound good!
Why are presets useful? Even really old organs have lots of sounds for each keyboard. Using good presets, you can go from one to the other by pressing only one button rather than a half dozen or so. If you have to press too many buttons when you change songs, you forget what you were trying to do in the first place.