The term 'wash' can mean several things:

Cleaning

A wash is the act of cleaning. I wash my hands in soapy water. See also: hand washing

Painting

A wash is a light covering of watercolor on a painting. When painting three-dimensional models, they can be used to add shading.

Geography

A wash is a small river or stream. Typically (at least in rural parts of the central United States, and the especially the deserts of the Southwest) the term wash is used to reference a stream that only carries water when there is rain, and usually dries quickly. The term does not refer to a section of land that gets wet during heavy rains, which is a flood plain.

Equality

There is a common American (also English?) phrase of, 'it's a wash'. This means the process of coming to be on equal footing with another party. If person A gives person B one dollar at the same time that person B gives person A a dollar, 'it's a wash'.

Brewing

A synonym for mash, a mixture of sugar and/or other carbohydrates (such as grains, corn, potatoes, etc), water, and yeast. The resulting mixture is fermented and then usually distilled to make vodka, whiskey, or rum.