It is a bubble level and inclinometer app which uses a smart phone with built in orientation sensor.
Hold your phone flat horizontally or vertically adjusting the bubbles between the center double lines.
Billiard tables, constructions, carpenter, You can use this leveler app anywhere.

Note:
Some phones will give different sensor readings based on the hardware used by the manufacturer.
And Some phone having a bad sensor(or error sensor) will show inaccurate result.