The specific meaning for home theatre purposes is a device that picks up radio broadcasts and allows you to select a certain frequency and thus a radio station. Tuners are usually included in the same box as a preamp and receiver, but not always.
Also an alternate word for a pitch pipe, a small harmonica-like device that one can blow on to produce an octave's worth of pitches. This is useful for tuning various percussion instruments, mainly tympani; though more advanced percussionists only need one starting pitch and can properly produce the other pitches on their own.