Answer Description
Accelerometers are used in many mobile devices (Android included) to track a device's motion and orientation. For example, an Android app that can be used to find a level surface is using the accelerometer to determine the devices orientation.
Wikipedia
An accelerometer is a device that measures proper acceleration Proper acceleration, being the acceleration (or rate of change of velocity) of a body in its own instantaneous rest frame, is not the same as coordinate acceleration, being the acceleration in a fixed coordinate system For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity, straight upwards (by definition) of g ≈ 981 m/s2 By contrast, accelerometers in free fall (falling toward the center of the Earth at a rate of about 981 m/s2) will measure zero Accelerometers have multiple applications in industry and science
Accelerometer - Wikipedia, the free encyclopedia