A synthesizer or other electronic instrument patch or program that was (most often) created by the manufacturer. Actually, many devices are shipped with presets onboard: effects processors, control surfaces, etc. Presets are often stored in ROM and cannot be overwritten. However, you can usually edit them to your liking and save your version in a user location.
Presets serve many useful purposes. First of all, they give you an indication of a particular piece of gear’s capabilities. They are often programmed by noted experts and sometimes even by “celebrities” in the field. Many musicians’ needs are satisfied by the available presets on a piece of gear and they find no need to edit. Others use presets as a jumping-off point for their own sound design adventures.