The resistor is an important electronic component because it reduces voltage.
It is like placing your foot on a water hose, reducing the amount of water coming out.
Resistors are found in most electronic circuits.
For example, most LEDs require 1.5V but if you were to use a square PP3 battery (which has 9V) you would blow the LED.
So a resistor is used to reduce the voltage down to 1.5V, protecting the LED from burning out.
A potentiometer is a special type of resistor that can be used to adjust resistance.
The volume control in a stereo system is a potentiometer.
The heat control slider on the thermostat in your house would be a potentiometer.
Using a potentiometer to change the brightness of an LED