A mash tun is a brewing tank used for converting and extracting sugars from grains and certain types of adjuncts. The crushed grains are loaded into the mash tun and then mixed with temperature controlled hot water. The hot water causes an enzyme reaction in the grains that converts their starches to sugars. The sugars are then rinsed from the grains with hot water that helps liquefy the sugars so that they can be more easily extracted from the grains. Many mash tuns are fitted with a raised perforated false bottom that permits the sugars to be extracted from the grains without requiring the grain husks to be transferred to the next stage of the brewing process.
The photo below displays a mash tun in the WestCoastBrewing.com home brewing sculpture/beer rack. The mash tun is in the center with the hot liquor tank to the right and the boil kettle to the left.