skimmer pool is also known as a freeboard pool. 

In this type of pool the water sits approx. 4 inches below the edge of the pool. So there is a gap between the top of the pool and the water.

A device called a skimmer is placed into the pool shell, below the pool edge. The skimmer draws the surface water into it, along with any dirt and debris. And then passes this water through the filtration system, before it is pumped back into the swimming pool.

How many skimmers are in the pool depend on the volume of the water to ensure correct turnover rate. Domestic pools usually have between 2-4 (up to 10 for commercial). 

The pool water will only be drawn into the filtration system at these points, rather than across the entire surface as they will be in overflow pools.