A preset is generically a setting or group of settings which can be recalled at will. The primary example of such a thing is the ability to store the frequencies of your favorite radio stations and recall them at will later simply by pressing a button or two. Pipe organs (and other organs) also have presets which can be used to change tones, and jump back and forth between them. Non-pipe organs use their presets to select between several unconvincing sounds such as brass, string, and wood. This latter type of preset cannot be altered.
The word "preset" is also used to describe factory (or default) settings, or options which have been set for you before you get some device (like a TV or stereo) or piece of software. These settings will typically be the ones desired by the largest number of users.