A refugium is a preditor free enviroment, that houses numerous macro algae's, live rock, and a live sand bed. It provides an excellent, natural way of filtering out nitrates and many other harmful chemicals. There is a large variety of things that can be kept here, all helping to filter naturally. Keeping these items out of the main tank, stops fish, crabs and other inhabitants of the tank from feeding on them, and allows them to grow freely. It also provides a degree of free floating food to the main tank. Hope this gives you a basic idea.