How the tuner hears a note
The tuner listens through your microphone and looks for the shortest stretch of sound that repeats, using the YIN method. One repeat is one period of the note, and the frequency is one over the period. It reads the fundamental, not the loudest overtone, which is what keeps a guitar's low E from being called an octave too high. The needle shows how many cents (hundredths of a semitone) you are from the nearest note, or from the nearest string when you pick an instrument. Within 5 cents is in tune: most ears cannot hear less.
Tunings and the A4 reference
Guitar is standard E A D G B E, bass is E A D G, ukulele is G C E A with the high G, and violin is G D A E. Press a string to hear its note, if you would rather tune by ear. A4 is 440 Hz unless you change it, anywhere from 432 to 446: orchestras often tune to 442, some to 443 to 446, and 432 has its own following. Every note moves with it.
Questions
Is the microphone recorded?
No. The sound is measured as it arrives and thrown away, in this page, on your device. Nothing is saved or sent anywhere. The microphone is only asked for when you press Start tuner, and stops when you press Stop or leave the page.
The tuner shows nothing. Why?
It needs a single clear note, a little louder than the room. Play one string at a time and let it ring. If the browser asked about the microphone and you said no, allow it from the icon at the left of the address bar and press Start again. A chord, or a lot of background noise, has no single pitch to find.
Can I tune a drop D or an open tuning?
Use Any note: it names whatever note you play and how far off it is, so you can tune to any note you like. The instrument presets show the standard tunings only.