refugium, not refigium (sorry to be pedantic, but misspellings bother me).
Anyway, it's a tank attached to your main tank in some fashion (either above, below, or to the side) that has no fish or large animals, but is populated with macroalgae, perhaps live rock & live sand, and detritus feeders such as worms, clams, etc. The refugium is lit, and has circulation. It acts as an additional biological filtration area where the algae, amphipods, etc. are not prey for the hungry critters in your main tank. The refugium may happen to grow phytoplankton & zooplankton which flow back into your main tank, or you may harvest the macroalgae from the refugium to feed your main tank (or as nutrient export) or you may cycle pieces of live rock from main tank to refugium and back to allow macroalgae and fauna to grow where otherwise, your tangs might munch it all away. (Whew, I may be bothered by misspellings, but not by run-on sentences!!! 8) )
