Everything sounds wrong on an instrument that’s out of tune, so this comes first, every time you pick it up. Tap Start, allow the microphone, and play a string: the needle shows how sharp or flat you are, and it reads “in tune” once you’re within 5 cents.
Pick your instrument (guitar, tenor banjo in G D A E, ukulele, fiddle or cello) and the tuner tells you which string you’re nearest and tunes you to that. Or leave it on Chromatic for any note at all: voice, whistle, mandolin, anything. It’s tuned to A = 440, works in the browser on your phone or laptop, and there’s nothing to install.