Basicly it's a little ro
big box that hangs on your tank that houses macroalgae and other critters that may not thrive or even survive at all in your tank. It's sometimes used to culture copepods, growing macro for food or to just filter the water, or to just place new arrivles in it before adding them straight to the actual tank. Others can be used as a small tank to keep little fish in if the fuge is big enough.
If you do get one building your own is usally the cheapest and you can customize everything to your liking.