a sump is a smaller tank usually located under your main tank, which is used as a type of filtration.
for example i have a 75g tank with a 30 gallon sump. water from the large tank goes into an internal overflow i have down into the sump. it flows over filter pads, bioballs, has a protein skimmer in it to suck out biological waste, and then its pumped back up into the main tank.
look up sump or wet dry trickle filter on youtube