A tuner detects and displays the pitch of the tone you a are playing, so it can be used to tune your instrument. A lot of tuners are electronic devices with a LED display to show the pitches on. Some electric or semi-acoustic guitars also have a built-in tuner. Nowadays there are also apps, like Karang, that can be used to tune your instrument. Tuner may also refer to a piano technician who tunes pianos or other keyboard instruments.