A metronome is a small machine — or now often a phone app — that clicks at a steady speed so a musician can keep time. The old ones had a swinging arm and a weight you slide: the beat gets slower or faster as you move it. Each click is one beat. Teachers set it to a number of beats in a minute. It does not tell you which notes to play; it only says how fast the beat is going. Though actually, many players practise with it and then turn it off, because real music slightly pushes and pulls the time. The word was built from Greek for “measure” and “rule”, which fits: it is a rule for measuring time in music. You will hear it in music lessons more than in ordinary chat, unless someone jokes that a person is too regular, like a metronome.