It measures the sharpness of the image on the ground glass and scores it while you turn the focus ring. It works, but it is the newest part of the app and is still being improved: the score can wander if the phone moves, and some phones do not let the app lock the phone's focus. Calibrate it once for each camera from the gear button, and keep the phone as still as you can.
There is more about it in How to Use, in the menu.
Choose your gear first. Camera, lens and film back feed every other screen: the apertures and shutter speeds on offer, frames per roll, depth of field.
Focus Score reads the ground glass through the phone camera and scores the sharpness while you turn the focus ring. It needs camera access and works best with the phone held still.
Nothing leaves your phone. No account, no tracking, no upload. Your rolls and photos stay on this phone, so keep a backup from Settings.
Place the phone over the finder, tap the spot you want to measure, then turn the focus ring. The green dot lights up at best focus.
Focus the camera first, any way you like: lens at infinity on something far away, or by hand until the ground glass is as sharp as it gets. The image on the glass must be at its sharpest when you start. Then hold the phone over the finder as you normally do and keep it still. The app tries every phone focus setting and keeps the sharpest one for this camera. It takes about fifteen seconds.