Idler pulley takes up the extra slack from the serpentine belt.Serpentine belt, drives accesories from the crankshaft ie., usually,. water pump, power steering pump, altenator,air conditioner compressor. Older vehicles had separate belts for driving these componants and each had it's own adjustment location which was done manually,[which is why belts would schreech when loose] most of the newer stuff has idler pulleys which are self adjusting [spring loaded]the bad part is serpentine belts can be pricey and the bearing in the idler pulley can tend to wear out and the whole unit has to be replaced. L. Keedy It controls the routing of the belt. The amount of contact betwen a pulley and the belt determines how much power will be available for the alternator, power steering pump or water pump. One of the idler pulleys is mounted on an arm that is spring loaded to keep force on the pulley. It's called a tensioner pulley or belt tensioner. That makes sure the belt is tight and able to transfer power properly.