Actually a really good question!
While most people think of an "aquarium" being the same as a "tank". In MOST cases that is true, but an aquarium really consists of the entire system. Point in fact, you can have (as I do) an aquarium that consists of both a display tank (or several) and a "sump".
So, "sump" is a part of your aquarium separate from the display tank that is used for many things: add water volume to the system, hide equipment, or house things like a trickle filter that requires gravity to feed it. It can also contain, or be a refugium.
A "refugium" provides a safe place, or a "refuge", for things that would be eaten in the main tank. This most often includes algaes or invertebrates. I have used mine to grow sponges to feed nudibranchs.
80 Gallon: Native Temperate Reef
5 Gallon Native Temperate Invert tank
20 Gallon Planted: Cherry Barbs, Common Pleco, Floating frog, Fire Bellied Newt
10 Gallon Planted: Apple Snails/Cherry Barb Breeder.... fingers crossed