Found in most electronic devices that have dials, a potentiometer is a variable resistor where the resistance changes as you turn the knob. In this linear pot the resistance is proportional to the amount the knob is turned going from 0 Ohms to 4.7k Ohms it can be connected to an analog input pin on a microcontroller to provide a smooth changing input value to control other devices like motor speed, light intensity.
VCC and GND are connected to the other pins and the centre pin will be the variable voltage. The pot is not designed to handle big loads like directly varying power to a motor. It is rated at 0.8W so is for use as an input device only.
See the datasheet for dimensions and further specs.