Refugiums are a separate habitat that is attached to your aquarium.  A good analogy is a business.  In a business you will have the front office and back office.   The front office is customer forward and looks great.  The front office is your aquarium.   The back office is where all the work happens to keep the front office looking good and producing a good product.  A functioning refugium will include organisms and systems that will

• consume nitrogenous waste, such as nitrate, nitrite, ammonia, and urea
• consume phosphate
• consume CO2 and produce oxygen
• add helpful bacteria