Resistance is how much a component opposes the flow of current. If the potential difference of the supply stays the same, then increasing the resistance in a circuit makes the current decrease. Decreasing the resistance gives a larger current. So a lower resistance means a larger current, and a lamp in a circuit with more resistance is dimmer.
A variable resistor is a resistor whose resistance can be changed. Many have a sliding contact that changes the length of resistive wire the current must pass through. A longer length of wire gives a higher resistance and a smaller current. A shorter length gives a lower resistance and a larger current. A lamp with a lower resistance in series with it is brighter.
A variable resistor is connected in series with the component it controls. It is used in dimmer switches and volume controls, and in experiments to change the current in a circuit and take a range of readings.
