What's My Sensor?
Accelerometer, gyroscope, light, pressure, step counter and the rest. What's My Sensor? lists each one your device has, shows what it is reading right now, and tells you in one sentence what it is for.
Open the app and it lists every hardware sensor your phone reports, motion, position and environment alike, with the name and vendor of each one.
Values update as you tilt the phone, walk around, cover the screen or step outside. Three axes where there are three, one number where there is one.
Each sensor comes with a one line explanation of what it measures, so a geomagnetic rotation vector stops being a mystery.
The app asks for no permissions, has no network access and shows no ads. Readings are drawn on screen and never stored or sent anywhere. That is the whole point: a window into your hardware, and nothing else.