Refugiums are basically a separate smaller tank connected to the main tank for pods and macro to safely grow free from predators. People usually put an ls bed, some lr, and various macro algea in them, or at least thats whats in mine. The pods and other critters just start growing on there own, you don't have to add them in.
The macro in refugiums removes nitrates from the water and can be harvested to provide food for fish. Also if your refugium uses a gravity return like mine some pods will make it into the main tank and provide food for the carnivorous fish.
They also provide more water volume which is always good, as well as more bio filtration from the ls and lr.