BPM is beats per minute, and a BPM metronome is a click that uses that number as its only speed. Divide 60 by the BPM and you have the gap in seconds. 90 BPM, the value filled in on this page, is a click every two thirds of a second. 60 is a click every second. 180 is a click every third of a second. The page accepts 20 to 300. Below that, the wait is so long that a missed click is hard to notice. Above that, the clicks run together and the accent is the only thing you can still count. The metronome schedules those gaps on the Web Audio clock, a little ahead of time, instead of waiting out each gap with a timer that can run late.
You can type the number or tap it. Tap tempo stores the time of each press, averages the recent gaps, and rounds to a whole BPM. Musicians name tempos as whole numbers, and a decimal here would pretend the tap was more stable than a finger is. If the taps are outside 20 to 300, the field is left as it was. Tapping does not start the sound. Start does, because the browser requires a gesture before audio. Once it is running, editing the BPM changes the following gap. The count of beats, shown above the buttons, keeps climbing from the moment you pressed Start. The accent is every first beat of the bar, using the beats-per-bar field, not a new bar each time you edit the tempo.
A tempo marking on a chart and a BPM in this box are the same kind of number only if you agree what the beat is. A piece marked 72 to the quarter is 72 in this box if you are counting quarters. The same piece felt in two is 36 if you set the click on the half note, or 144 if you set it on the eighth. The page cannot see the chart. It clicks whatever number you give it, once per beat, with no eighth-note grid inside the beat. Doubling the number is how you hear the division. Setting beats per bar to 3, 4, or 6 only changes which click is higher.
Common anchors, not recommendations: 60, 72, 90, 108, 120, 144. Walk between two of them and the difference should be obvious. If it is not, the tab is muted or you are hearing something else. Keep this tab in front. Stop, or closing the page, is the end of the schedule. Nothing is saved. The online metronome page is this same click, described from the point of view of not installing an app.