A refugium is a separate tank where most people keep delicate species of invertebrates, as well as some saltwater plants, such as caulerpa (as well as many other different types). The purpose of keeping plants is for them to help in maintaining good water quality (plants absorb nitrates, phosphates, etc.) However, the catch is that this separate tank is still connected to the main tank through water flow. Water is pumped through the refugium and back into the main tank, just like having a wet/dry filter.
The whole idea of the refugium is to be able to grow plants and not have reef inhabitants eat them, and it is also where most people grow their pod population for such fish as scooter blennies and mandarins.