Fuge is sorta slang for refugium, which can be a section in your sump, or an add-on reservoir to your main tank. Basically it's a place where you can add live rock/sand and grow micro-fauna and macro-flora without the possibility of something in your tank eating them. Many people grow chaetomorpha (a macro-algae) that can help reduce your nitrates and phosphates by consuming them as water passes through the sump, and also breed copepods, which can be added to the tank as additional clean-up or live food for other fish.