A sump as a location of external filtration for your tank...the water is removed from the tank, flows to the sump, goes through the various stages of filtration with in the sump...most people whom use refuges do so for additional filtration...generally the water flow into and out of the refuge is somewhat slower the the normal sump flow...some will have a seperate pumping system taking water from the sump into the refuge and back into the sump...others will have the overflow (water from the tank to the sump) valved off to supply a slower flow rate directly to the refuge and then to the sump.
Do a search for sumps here...they can literally be however you want them to be... limited to your own imagination.