Sump is another small tank (Some people use Rubbermaid containers) that sits under your main tank. Water is drained to the sump via an overflow box that sits on the top edge of your tank. If you have a tank that is drilled and has a built in overflow, your don't need the overflow box. You need a pump in the sump to return water to the main tank. Purpose of sump is to add extra water capacity to your system, to increase water flow through your tank; also a place to put equipment- skimmers, heaters, filters.