Pond skimmers are buried by the side of your pond behind your liner. The liner then attaches to the opening of your skimmer with silicone, included plate, and screws to prevent water from leaking out around the edges of the skimmer. The pump (sometimes enclosed in a bag or pump defender to filter out finer particles) within the skimmer pulls water and debris from the surface of the pond into the skimmer's opening. A plastic basket inside the skimmer collects leaves, pollen, twigs, uneaten food, and other floating objects. You'll empty that basket on a regular basis as part of pond maintenance. Most medium to large ponds have both a skimmer and other types of filtration units.