Calurpa is a type of MacroAlgae that "eats" nitrates. There are many types of calurpa (sp).
An algae refugium is a place where you can grow various algea types - usually calurpa away from the main tank. This refugium can literally fill with algea that feed off nitrates. You periodically "harvest" the algae, giving some of it away to your fellow tank keepers

, throw it away or trade it to your nice LFS where they will give you a store credit.
The reason you do it in a refugium is to keep your cleanup crew from eating it, and you can control the lighting separately (run 24hours, opposite of your tank lighting for PH stability, etc.)
HTH