Preset is a small variable resistor generally used to provide a variable resistance in a circuit in order to tune the circuit. They are directly mounted on the PCBs. A screwdriver or similar tool is required to adjust them.
A 20K-ohm metal preset is shown in the image above i.e. the maximum value of resistance that we can give in a circuit by using this preset is 20k ohm.
The rectangular space between the cover of the preset is used to rotate it on the track. Upper side of the cover has a slight curved dent which connects the resistive strip with third leg (upper leg) of the potentiometer.