A metronome marks time so you can practise against a steady pulse instead of against your own rushing. This one is an online metronome: the click is made in the browser with the Web Audio clock, and the only control that matters is the BPM, the beats per minute. At 120 BPM there are two clicks a second, and the gap is exactly half a second. At 60 BPM the gap is one second. The page will not accept a tempo under 20 or over 300, because outside that range a click stops being a musical pulse and becomes either a drone or a buzz. The BPM metronome page is the same tool with the tempo explained from the number rather than from the start button. The online metronome page is the same tool again, for the search that already says online.
The scheduling is the part that makes a browser metronome usable. A loop that waits "about 500 milliseconds" and then beeps will wander. Timers in a browser run when the page has a turn, and a busy tab, a scrolling article, or a laptop saving power will hand that turn over late. The click here is not fired when the timer happens to run. The timer only looks ahead. Every 25 milliseconds it asks the audio clock what time it is, and if the next click falls inside the next 150 milliseconds, it schedules that click on the clock at the exact future instant. The clock then plays it, even if the next timer is a little late, as long as the lateness is inside that window. The gap between scheduled times is 60 divided by the BPM, not a fresh guess each beat. Change the BPM while it is running and the next gap uses the new number. The beats already scheduled are left alone.
Beat 1 of the bar is higher, 1760 Hz against 988 Hz for the other beats, and a little louder. Both are short square-wave ticks, about 40 milliseconds, which reads as a click rather than a note you would tune to. Beats per bar runs from 1 to 12. Four is the usual setting: strong, weak, weak, weak. Three gives a waltz. Six is there if you count a slow 6/8 in six, though many players will set the BPM to the dotted-quarter and use two or one instead. This page does not subdivide. It will not click eighth notes between your beats, and it will not swing. If you need a grid inside the beat, double the BPM and decide which of those clicks you treat as the beat. That limit is stated here so a practice session is not built on a feature the page does not have.
Tap tempo is the other way in. Press Tap tempo with the music, or with your foot. The page keeps the last few tap times, averages the gaps, and writes a whole number into the BPM field. Two taps produce a tempo; a few more pull a sloppy first pair back toward the pulse you meant. A gap that works out under 20 or over 300 is ignored rather than stuffed into the field. Tapping does not start the sound. You still press Start, because that is the gesture the browser requires before it will let a page make noise. If Start does nothing audible, the tab may be muted, the output may be a disconnected Bluetooth device, or another application may be holding the sound exclusively. The lamp in the circle flashes with each scheduled click. It is a visual check, not the timekeeper. Trust the sound.
A few tempos people actually set. 60 is one a second, useful for counting in slowly or for a pendulum comparison. 72 and 80 show up in marches and in a lot of pop ballads. 100 to 120 is where a great deal of practice material sits; 120 is the default in the box so the first press is a recognisable pulse. 132 and 144 are brisk. 160 and above is fast enough that the accent matters more, because the unaccented clicks start to blur. None of these is a recommended song tempo. They are anchors so you can hear whether 108 feels different from 120, which it should. If two settings sound the same, you are not hearing the clock; check the mute switch before you blame the maths.
The sound stops when you press Stop, when you leave the page, or when the BPM field becomes something the page refuses. There is no account and no recording. The microphone is not used; tap tempo is a button, not a listen. Keeping the tab in the foreground is the practical requirement. Phones are aggressive about background tabs, and a metronome you cannot hear is not a metronome. The wheel is unrelated if you were picking a practice order; this page only clicks. It is not a tuner, not a backing track, and not a substitute for playing with other people, who will drift in ways a clock will not follow.