Pulleys are a simple form of machine. They make jobs easier to do. Pulleys are usually used to lift things.
If we ignore friction the more pulleys in a pulley system the less effort you have to use to lift up a certain load.
The pulley system shown in this animation has two pulleys. The upper pulley is fixed to a support while the lower pulley is free to move up and down.
If the effort on the right hand rope is pulled down 1m then clearly 1 m of rope must come over the top pulley.
This means that EACH of the ropes holding the bottom pulley must shorten by ½ m and so the load rises ½ m. This means that the velocity ratio of our pulley system is 2. However if the pulley is 100 % efficient and the velocity ratio is 2 then the mechanical advantage is also 2 and that means that the load is double the effort. This means, for example, that you can lift a load of 100 N using an effort of only 50 N.
If the pulley system is perfectly efficient the mechanical advantage and the velocity ratio are both equal to the number of pulleys (in this case two).
However no pulley system will be 100% efficient because not only will there be friction in the axles but the pulleys themselves have weight and also need energy to be lifted.