The more resistance something has the smaller will be the electric current flowing through it for a certain voltage. The resistance of a circuit can be changed by altering the components in the circuit or simply by adding more wire.
One way of increasing the length of wire in a circuit is to use a device called a variable resistor or rheostat. This is made of a length wire wrapped round a former. It can either be a straight as in the picture or wrapped round into a circle.
If a potential difference is connected between terminals B and C a current will flow through the wire. If the slider is moved towards the left hand end of the coil of wire this current will have to flow through more wire and so the resistance of the rheostat in increased and the resistance in the circuit will also be greater.
If a voltage (say 6v) is connected between terminals A and B and an output taken between terminals A and C this output can be made to vary between 0 (slider at the left) and 6V (slider at the right). This device is now called a potential divider.