what is a refugium?

We actually cover this under the filtration section of the getting into saltwater article (https://aquariu.ms/article/6-salt…0657-filtration)

But in short, they are just another filtration method that can be also used to source in live food. “A refugium typically contains aragonite live rock, macroalgae, and a deep sand or mud bed and tends to be the best natural way to filter out your tank (as it would be done in nature without any mechanical aspect). This works by allowing for particles, uneaten food, and debris to settle on the sand or mud bed where micro crustaceans live and break down these aspects while also providing an additional food supply when needed.”