Spirit Level. Depending on inclination, either bulls eye level with roll & pitch gauges OR horizontal and vertical bubble levels are shown. Calibration feature to correct for accelerometer misalignment & different axis sensitivities.
For best results touch CAL and calibrate first. The Spirit Level is only as good as your accelerometer. For indication only. Don't damage your tablet or phone, use a real spirit level when appropriate.
Pause button to hold the current reading.
Lock button to stop from automatically changing screen with inclination.
Zero and reset button to make measuring angles easier.