A hopback is a sealed chamber that is inserted in between the brewing kettle and counter-flow wort chiller. Hops are added to the chamber, the hot wort from the kettle is run through it, and then immediately cooled in the wort chiller before entering the fermentation chamber.
It facilitates maximum retention of volatile hop aroma compounds that would normally be driven off when the hops contact the hot wort. Because it is a sealed chamber, the volatile hop compounds are trapped in the hot wort, and then the wort is immediately cooled, which keeps the volatile compounds in solution.