its just a section of the sump or even tank that is used to house things , it could be housing macro algae(seaweed) which uses nutrients like nitrate and phosphate as it grows and is harvested periodically to export the nutrients from the tank , this type of set up is normally lighted and on a reverse cycle to that of the main tank. it could also have some pieces of live rock in it to allow pods to grow and multiply safe from predators( some will still make it to the tank) also it could be used to house a DSB (deep sand bed) which is just a thick layer of sand deep enough to support anaerobic bacteria which complete the nitrogen cycle, turning nitrate into nitrogen gas, while also housing many beneficial creatures which turn the sand over . a dsb is normally used separately to a refugium but may incorporated as may the macro and live rock in combination