What is a pulley?
A simple pulley is a system that uses a wheel and a rope. The purpose of a pulley is to make lifting things easier by changing the direction of effort, because pulling down is easier than lifting something up.
A fixed pulley is a pulley that can not be moved to a different location (it is stationary). It does not reduce the effort needed to lift a load, but it does make it easier (more convenient) to life something because it it changes the direction of force.
Here are the parts of a pulley.
When multiple pulleys are added into a system it reduces the force required to lift the load by spreading out the force over multiple strings. This is known as mechanical advantage.
Rock climbers also use pulleys and mechanical advantage! One person helps lift the climber from the ground and suspend them on the wall as they climb. It could be very tiring to hold the weight a another human for a long time if they did not have a pulley system to reduce the force needed and change the direction of the applied force. Imagine if you had to just pull a rock climber from the top of the wall-that would be a hard job!
Find out how Science Max resolved his roller problem by watching this video from 12:39 until 16:04: