They are devices that take very small electrical signals and bring them up to a standard level so that they can be fed into standard devices for recording, amplifying to audible levels, and other such manipulation. Microphones require preamps. The cartridges from phonographs too. Guitar amps contain a preamp stage and a power amp stage. Preamps meant for microphones tyically also contain a source of so-called phantom power for condenser mics that require it.