A sump is a low area that collects an often-undesirable liquid(s) such as water or chemicals.

An example is the oil sump of a gasoline engine. The oil is used to lubricate the engine's moving parts and it pools in a reservoir, known as a sump, at the bottom of the engine.

Another example might be the lowest point in your basement into which any water that seeps into your basement flows. If this is a regular problem, you may have a sump pump that moves the water outside of the house.