The Reinheitsgebot is the German beer purity law. It is held to have come about in the year 1516 and states that beer can only consist of malted barley, hops, and water. With the discovery of yeast in the 1800s, it was amended slightly to account for this necessary microorganism.
Many brewers tout their beers that adhere to the reinheitsgebot. It means that no adjuncts like corn or rice were used in making the beer. However, more "desirable" adjuncts—oatmeal in an oatmeal stout for instance—are also not allowed.
While German brewers can indeed use adjuncts in their beer, they cannot label such beers as lagers for sale in Germany.