A level switch is a sensor that detects the presence of liquids, powder, or granulated materials at a specific location.(It is also referred to as level sensor).
For example, when a tank is supplied with liquid, it overflow. In order to prevent overflows, a level switch is installed near the top of the tank.
When the liquid touches the level switch, it outputs a signal to keep the liquid from overflowing the tank. (To detect that a tank is filled with material is referred to full detection, or upper limit detection.)
On the other hand, when the liquid level drops as the liquid is discharged, a level switch is used to prevent completely draining the tank. This is referred to as an empty detection or lower limit detection.