Mash is a way to say a recipe of corn, wheat, or rye and malted barley that are combined with heat and water to make a mixture.
The mash bill of a Bourbon is its recipe. Bourbon contains at least 51% corn. Any grain is possible; some distillers use rice, oats, and other grains. Most Bourbons are a mix of corn, wheat and/or rye, and barley.