Fork listens through your microphone and names the note you are playing. The letter settles, the dot centres and the screen goes still. That stillness is how you know you are in tune.
It works on guitar, bass and ukulele, where it picks the string you are closest to and shows how far off you are in cents. In chromatic mode it simply names whatever it hears, which is what you want for anything with no fixed tuning.
Nothing is uploaded and nothing is stored. The pitch detection runs entirely on your own device, so the audio from your microphone never leaves the page.