A movable pulley is a way to use a pulley to make it easier to move heavy things – to get a mechanical advantage. In the picture, someone is trying to pull their bike up to the ceiling so it will be out of the way. They’ve used two fixed pulleys, attached to the ceiling, and two movable pulleys, hooked on to their bike.
The fixed pulleys make it easier to pull the bike up, by changing the direction of force so he can pull down instead of having to lift the bike up. But the movable pulleys give him a mechanical advantage too:
Here’s a video of someone actually using a bike hoist: