By definition it is a 'safe place'. The nick name is fuge. In reef tanks it is usually a place to grow macroalgae that will take up excess nutrients and therefore help control nuisance algae. It is usually setup as a section in the sump with a strong light over it. Chaetomorpha (commonly called chaeto, pronounced 'ket-toe') is considered the ideal macroalgae by most because it cannot turn sexual like Caulerpa spp. can. Some people use it to grow pods
as a food supply for mandarins, but I can't think of a single tank that was able to support a mandarin after adding a fuge when it couldn't before.