A sump is basically a support system beneath your main tank where various equipment and filtration systems are placed [out of sight] that are geared to support the main aquarium. In its simplest application the sumps can be a 10 gal tank or even a Rubbermaid container where equipment such as skimmer, heater, pumps, etc., are placed so that these items do not clutter up the appearance of the main setup. More elaborate sump configurations can include separate ecosystems that are also designed to support the main tank, such as a refugium [algae tanks, mangrove tanks, LS pod tanks—anything really you want to add in there that is designed to support your main tank]. The water between main tank and sump is exchanged by hoses or pvc plumbing with water leaving your tank, emptying into the sump and then returning back to your main tank. Some more expensive tanks come “pre-drilled” with the basics of this plumbing already in the design of the main tank; otherwise, hobbyists that are diy inclined, can rig these systems up themselves, modifying them at will. The use of the sump can basically get your hands and aquarium equipment out of your main tank making it easier for maintenance purposes, etc.
Here is an example of our own Mr. Salty’s sump beneath his aquarium
<a href="http://mrsalty.homestead.com/equipment.html" target="_blank">Mr. Salty's Tank</a>