Set a tempo or tap one in, pick the time signature, hit Start. It runs on the browser's audio clock, so it stays steady even when the page is busy.
A metronome is great for practice. On stage, a lot of bands can't run one at all: no in-ears means nobody can hear the click, and plenty of drummers would rather push the chorus anyway. If your keys, delays or loops still need a tempo, DrumClock flips it around. The drummer counts it in, and the gear follows the drummer instead of a click. More on that: playing without a click track. Need delay times in ms? Delay time calculator.