What does "keep time" mean?

Maintain the correct tempo and rhythm of music; also, mark the rhythm by foot-tapping, hand movements, or the like. For example, The children love to keep time by clapping their hands. This usage dates from the late 1500s and is occasionally put figuratively, as Ben Jonson did in Cynthia's Revels (1699): “Slow, slow, fresh fount, keep time with my salt tears.”

Also, keep good time. Indicate the correct time, as in This inexpensive watch does not keep good time. [Late 1800s]