Catalyst that allows your beer yeast to ferment more malt sugars out then it usually can. This produces a crisper, drier or less sweet beer. It will also increase the final alcohol strength if you start the beer at its regular OG.
Add at the start of fermentation when adding yeast. Most standard beers will ferment down to 1.000-1.002, All-malt beer kits will ferment down to approximately 1.006, Allow for a longer fermentation time.