TunehoffLite needs microphone access. Open Settings → Privacy & Security → Microphone and turn on TunehoffLite. The status pill at the bottom of the screen shows Mic access off or No microphone when it can't hear you.
The big letter is the note the string should be. The number is how far off you are in cents: − means flat (tune up), + means sharp (tune down). The top of the screen says Tune up, Tune down or In tune.
Tap the gear icon in the top-right corner.
Tap a string button (E A D G B e) to lock the tuner onto that string. Tap it again to go back to automatic detection.
Email miles@qrfoil.com — include your iPhone model and iOS version if something isn't working.