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 animation shows a pulley system with three pulleys. if the effort on the right hand rope is pulled down 1m then 1 m of rope must come over the top pulley. This means that EACH of the ropes holding the bottom pulley must shorten by 1/3 m and so the load rises 1/3 m. This means that the velocity ratio of our pulley system is 3.
However if the pulley is 100 % efficient and the velocity ratio is 3 then the mechanical advantage is also 3 and that means that the load is treble the effort. This means, for example, that you can lift a load of 100 N using an effort of only 33.3 N.
If a pulley system is perfectly efficient the mechanical advantage and the velocity ratio are both equal to the number of pulleys.
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.