The Varaible Resistor is just that, it is a resistor that you can vary. This is also called a potential divider or pot for short. The one in the photo has a slot in the top and three pins underneath. It has a maximum value of 10,000 ohns (10k) and a minimum of 0 ohms.
The pins underneath split or divide the resistor. You connect GND (0 volts or negative) to one of the pins at the front (in this photo) and Vcc (5volts or positive) to the other one. The pin at the back (effectively the middle) divides between the two.
So as you turn the pot it goes from 0volts to 5 volts and back again depending which way you turn it. A bit like the volume or brightness control knob.