The level is a measuring instrument employed to determine the slope of a surface with respect to a reference horizontal plane.
There are various types of levels. The most common one is the bubble level, whose operation principle is that filling a container with a liquid and leaving an air bubble inside, the bubble will be positioned always at the highest point of the container, because the air is less dense than fluid. Therefore, if you fix the ball to a device that you desire is perfectly levelled, you have to move the bubble level until the bubble will be at the top of the sphere.
Another type of level is the electronic one, which allows a highest precision measurement and offers the possibility to read, with precision and in real time, the detection on the display.