Hey drumbum - welcome to Fish Lore.
Refugiums are separate yet connected bodies of water that allow you to grow macroalgae and pods for the main tank. The macroalgae can be used to export excess nutrients coming from the main tank such as nitrates and phosphates. Whenever I prune the chaeto and caulerpa from the fuge there are tons of little pods in there. These pods make their way into the main tank where they become food for the fish.
You can definitely make one yourself inexpensively. I made one out of an aquaclear powerfilter. Is it going to be as good as a larger fuge? Prolly not, but still worth it in my opinion.
Link:
Hang On Tank Refugium
Also, plan on getting some
live rock too.