Yes Oceangrl is right. Basically a refugium or fuges it is called is an "isolation zone". A safe haven for plants. Normally in the main tank with fish plants would be eated by some fish and inverts. This way with a refugium plants can benefit the main tank without being effected by the inhabbitants of the main tank killing the plants. I perfer my sandbeds to be 4" but others like 3" but it's all up to you. Just make sure that the fuge is tall enough for a dsb (deep sand bed) and the growth of your plants. Also of course you need decent lighting in order for the plants to grow I suggest no less than PC lights (Power Compact Fluorescent Lights). My refugium currently is only 4.7 gallons which should be good enough the "clean" a 40 gallon tank. Although my tank is 40 gallons I will upgrading to a refugium that is nine gallons (the actual spot for the plants and etc. is 9 gallons but the whole thing is more). Along with the sand you might want to add some live rock "rubble" or pieces that were broken of of larger pieces, rubble is usually not as "pretty" but it doesn't matter. Also along with the sand and rock rubble you should get animals for the refugium. You must make sure that they aren't going to eat your plants. Here are a few of the animals you can add to a refugium...
-Sand Burrowing Brittles Starts (small ones that grow no larger than get a disc no bigger than 1/")
-Nassarius vibex
-Turbo Snails (I added one Turbo from my main tank and it is doing a good job on the algae that grow in the refugium but leaves the macro algae alone)
Okay now on to what is macro algae? Macro algae is the "good" algae. These are such plants as caulerpa, shaving brush plants, mermaid's fan plants, halimeda, Tang Heaven, etc. These are the algaes you want to grow in your fuge. You will want these plants to fill the fuge and when they grow to big prune them and you can throw the clippings into your main tank and the inverts/fish will eat them.
I believe refugiums are great to have and almost everyone should have one but it must be said they are NOT required but are very handy. Good luck! Happy fishing!
:happyfish