A guitar tuner is a device used to ensure that a guitar's strings are tuned to the correct pitches. It detects the frequency of the sound produced by each string and indicates whether it is sharp, flat, or in tune. This helps musicians achieve the desired sound and maintain harmony while playing. There are various types of guitar tuners, including clip-on tuners, pedal tuners, and app-based tuners. Clip-on tuners attach to the headstock of the guitar, while pedal tuners are used in live performances. App-based tuners can be downloaded on smartphones, providing a convenient option for tuning on the go.