A pool skimmer is a device that removes contaminants from the top layer of a body of water. At the front part of a skimmer there will be a floating door or weir, the weir's job is to restrict water flow into the skimmer and only allow the top layer of water in, carrying the contaminates with it.

A weir also amplifies the suction of the skimmer to the top part of the water only, increasing its effectiveness.