This is the German (originally, Bavarian) purity law that restricts the ingredients that can be used to make beer to being water, barley malt, hops, and yeast. In the 1516 version of the law, only water, malt and hops were mentioned, because yeast was not isolated until the 19th century by Louis Pasteur. The Reinheitsgebot is actually part of a larger document called the "Biersteuergesetz" or "Beer Tax Law" which defined what beer was and how it should be taxed according to strength.

"Rein" means clean or pure; "-heit" means "-ness"; so "Reinheit" means "cleanliness" or "purity".

In 1987, the Reinheitsgebot was repealed by the EU as part of the opening up of the European market. Many German breweries elected to uphold the Reinheitsgebot in their brewing anyway, out of respect for their craft and heritage.