Mashing is the process of combining grains with water and then heating the mixture.
Mashing allows the enzymes in the malt to break down the starch in the grain into sugars, typically maltose, to create a malty liquid called wort. In large distilleries there is at least one dedicated vessel for mashing, called the Mash Tun. Some have dozens in operation at any given time.
Mashing is also the stage that grains are sometimes mixed together, and typically the likes of malted barley is supplemented by grains such as corn, rye or wheat. This is referred to as creating a "mash bill" (creating a mash recipe).