A bubble level, spirit level or simply a level is an instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb).
Bubble Level app is a useful tool for your Android device.
Your device should already be calibrated by the manufacturer. In case you believe it is wrongly calibrated you can re-calibrate your device by pressing calibrate button. Placing your device screen facing up on a perfectly leveled surface (like the floor of your room) and press CALIBRATE. Press RESET to return to your device default factory calibration.