What a mic test can and cannot tell you
It answers three questions, and they are the ones that actually go wrong before a call or a recording.
Is the right device selected? If the meter stays flat while you talk, the browser is listening to something else: a disconnected headset, a monitor's built-in microphone, a virtual device left behind by some software.
Is the level sensible? Too quiet and you will be amplifying noise later. Too loud and you are clipping, which is unrecoverable.
Is the noise floor acceptable? What the meter shows when you are not talking is what will sit underneath everything you say.
A mic test answers those three and stops there. What it cannot tell you is how you sound. Level is not tone, and a microphone can be perfectly set up and still be the wrong distance away or pointed at a hard wall. For that you need to record something and listen, which is what the voice recorder is for.
Reading the numbers
The scale is dBFS, decibels relative to full scale, where 0 is the loudest value the format can hold and everything below is negative.
Aim for peaks between −6 and −12. That is loud enough to sit well above the noise floor and quiet enough to leave headroom for a laugh or a raised voice.
Averaging around −18 to −20 while speaking normally is healthy. If your peaks are hitting −30, the level is too low: you can fix it in editing, but you will bring the room noise up with it.
The one mistake a mic test cannot warn you about afterwards
If the meter's top bars light red and the warning appears, the signal has reached 0 dBFS. At that point the waveform is not merely loud, it is flat-topped: the peaks have been cut off and the information in them no longer exists.
No amount of editing brings it back. Turning it down afterwards gives you a quiet distorted recording rather than a clean one.
So when in doubt, record quieter. A recording peaking at −18 is trivially fixable and a recording that clipped is not.
What the meter does not do
There is no recording, no buffer and no upload in this mode. The audio goes from your microphone into an analyser node that reports amplitude, and nothing retains it. Press stop and the track is released, which is why your browser's indicator goes out at that moment rather than lingering.