Merriam-Webster defines a sump "as a pit or reservoir serving as a drain or receptacle for liquids" and "as a pit at the lowest point in a circulating or drainage system". In the saltwater arena, the sump is used to increase total water volume and to house items that aren't wanted in the main display tank (heaters, skimmers, pumps, etc.).
Often sumps are located in the stand or in a room behind the tank. In my case, when we decided to upgrade to the larger tank, I built a large shelving unit in the basement, directly below the tank, and relocated the sump. After moving the sump, we also decided to add separate GFCI outlets and switches to control the various electrical components located in the sump.