A pulley is a small wheel with a groove around its edge. The groove can hold a rope in position.
As the wheel rotates, the rope moves in either direction. It makes lifting of objects, easier. A pulley changes the direction of the lifting force. That means, when you have to lift a heavy object, you pull down the rope and the object lifts up. Pulleys are used for drawing water from wells, hoisting flags and to lift very heavy objects with cranes.