A skimmer is a plastic box mounted outside the pond and accessed through a hole the liner. It is underground and mounted just at water level. They usually contain both a pump and filter media. The purpose of it is to suck leaves, right into skimmer instead of letting them settle to the bottom of your pond. The leaves and debris usually sink to the bottom of the pond before they flow into the skimmer, so I advocate not buying them. They cost a bunch of dollars and require a huge hole through the liner that has a tendency to leak after a few years and can give you all kinds of problems you don't want.
They do provide water level safety not letting the pump pump all the water out of the pond if there is a waterfall problem, because they are up from the bottom by 2 ' inside the skimmer.
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