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yourmother The accelerometer, gyroscope, compass, barometer, thermometer and any other sensors present on a given device. They're used by some apps, for example some games, in navigation apps, etc.
From the documentation on the GrapheneOS website:
Sensors permission toggle: disallow access to all other sensors not covered by existing Android permissions (Camera, Microphone, Body Sensors, Activity Recognition) including an accelerometer, gyroscope, compass, barometer, thermometer and any other sensors present on a given device. When access is disabled, apps receive zeroed data when they check for sensor values and don't receive events.