A chromatic tuner is a small electronic device used by musicians to tune instruments. It consists of an input facility (small integral microphone and/or jack socket), a display and an optional output jack socket. Usually powered from a PP3 9V battery.
It displays the name of the note you are playing with some indication of how out-of-tune it is and in which direction. It is called a chromatic tuner because it detects and displays any note (see chromatic scale). More restricted, usually cheaper tuners only detect certain notes, such as EADGBE for a guitar tuner.
The reference pitch is often selectable, allowing the user to tune away from the standard concert pitch (A=440Hz). The adjustment is sometimes restricted to a selection of frequencies, but better tuners allow teaching of a reference note as the standard. For example, if you want to play along with a piano that is three-quarters of a semitone flat, you can play an A on the piano and tell the tuner to use that as the reference.
If you own a tunable instrument and play with other musicians, get one.