Pulleys are pretty advanced bits of outdoor climbing kit and there are two main types. There are simple pulleys, and these are used to reduce friction on the rope passing through them, these are mainly used when hauling gear up or down the route. There are also auto-locking pulleys and these can lock out if rope moves through them too quickly. This is useful for hauling heavy gear as you can pause and the pulley will lock up, taking the weight while you rest or need to move to a better position. The second major use of locking pulleys is in self-rescue situations, this is a very advanced skill-set and the self-locking pulley will help you climb up your rope. The third use is for simul-climbing and self-locking pulleys can be an extra safety device to take the strain from the lead climber in case of a fall.