A refugium is a 'refuge' in the sense that while it shares the water of the main tank, it is somehow set off from the tank to provide the creatures inside refuge from larger fish in the main tank. Another word, a refuge is an extra tank or compartment of your tank. It can also house any aggressive fish that's in your tank. With additional lighting, it can be used to grow algae to harvest. Growing algae in a sump/refugium also help absorb waste that would otherwise pollute the main tank.