A skimmer is a necessary element of keeping a pool clean. The skimmer sits at the waterline and catches leaves, bugs and trash so they do not clog the filter. Skimmers with a second hole help keep the pump from running dry if the water level is unstable.
The first hole in the skimmer leads to the swimming pool pump. When the pump is on, water is drawn from the pool into and through the skimmer taking debris with it. The skimmer basket traps debris so it does not clog the drain.
The second hole in the skimmer is connected to a pipe called an equalizer. This pipe goes down approximately two feet and through the concrete wall of the pool. It prevents the pump from taking in air if the pool's water level dips below the skimmer opening. If the pool water is low the pump pulls water from the equalizer line so the pump doesn't run dry.
If the skimmer is not working properly, your pool will get dirty and the filter will get clogged. Clean the skimmer basket at least once a week and more often if there are a lot of leaves dropping into the pool. Do not use a skimmer that is broken; replace either the broken parts or the whole apparatus.