A potentiometer henceforth referred to as a pot, is a variable resistor. It is an electrical component with three terminals (i.e. pins or legs): one for input voltage (e.g. 5V), one for output voltage (e.g. GND) and one for picking up the value of the pot (we’ll call this the wiper).
Mechanically speaking, pots are physically adjusted usually using your fingers. Both rotational and linear pots are common.