A mixture of malted barley and flaked wheat, along with small amounts of flaked oats and acidulated barley. This goes in the oven, where it is held at 150 degrees for an hour. If all goes well, enzymes from the malted barley will convert the starches to sugars, producing a wort which can then be fermented into beer. The trick is to keep the temperature exactly right: too cold and the starches won't turn to sugar, but anything above 170 will kill the enzymes and ruin the mash. Acidulated barley is grain that has been allowed to go sour (basically partially rotting it to produce lactic acid) which gives that yummy sour Belgian taste.