I spose its just another tank u pump ur water into, filter it, heat it, uv
whatever, and then pump back into ur main tank..
Refugium mostly a part of the sump, place to grow live rocks, i spose plants, also helps with filtration to run water through alot of LR
, put all ur hardware into the sump to keep ur tank clutter free.
it could be explained better, i'm sure many will get a bit more into it..