Refugiums are an area of your system that has no predators of whatever your trying to grow in your 'fuge. (i.e. 'pods, macro algae, something else) They take refuge there, hence the name, and multiply unhindered. Some people build them into their sump, but I'd say a separate one is better, that gravity feeds back into your tank to provide whatever you were growing there a return trip that isn't aided by a big return pump. (slice and dice action on the way through)
I'd recommend doing some searches on here by going to google.com and typing in:
site:reefcentral.com build refugium
or
site:reefcentral.com setup refugium
or
site:reefcentral.com sump planning
That will search only the RC site for the information you are looking for.