Tempo is the Italian word for time, and we use it to describe how quickly a piece of music is performed. We measure tempo with Beats Per Minute, or BPM. If something is defined as 60 BPM then we know there is a beat every second, and if a piece of music is set to a specific BPM then those beats will strictly fall to the selected time.
A metronome is a device used by musicians to maintain a steady pace while playing a musical piece. This pace (tempo) is adjustable and the device produces an aural beat. It is of utmost importance to learn to keep to the pace of a metronome.
These devices are available in Analog and Digital versions. They are also available as apps for android and IPhone.