A refug is a broad term but generally it is a place in the sump thats used to grow macroalgae and/or have a DSB(deep sand bed).DSB's are great for nitrate reduction.If you plan on haveing a DSB in your display tank,then its not necessary to have one in your refuge IMO.The refuge is kept free of fish,corals,crabs,predators.This gives copepods and amphipods a place to populate and reproduce at.Pods(slang term)are great in a reef tank,they graze algae as well as provide a food source for fish/corals.