It's a sort of strainer. This one's on castors, so it can be used as an under-back beneath the mash tun then a hop-back beneath the copper. A hop-back acts as a reservoir which is filled with fresh hops before the flow from the copper is started. The hot wort flows through the fresh hops and is quickly chilled by a counter-flow of cold water before entering the fermenter. Many of the volatile hop aroma compounds are extracted and brought into the fermenter with this process. It is reckoned to produce a flavour/aroma profile somewhere between late hopping in the copper and dry-hopping in the cask.
Barrie told us that many people thought his beers must be dry-hopped.