High Gravity, when used to describe a beer, refers to the specific gravity of the wort, which is what beer is called before fermentation. When the wort's specific gravity, aka the Original Gravity, exceeds 1.070, the finished product is called a High Gravity Beer. The significance of High Gravity is that more sugars and flavoring agents such as caramel and dextrins are present in the wort, yielding a more alcoholic and flavorful beer.
Stout and Porter are examples of High Gravity Beer.
by Das Brau Meister October 27, 2013