A flat wash is a basic watercolor technique where a large even area of color is applied. This concept is one of the base building blocks of watercolor painting.
A flat wash is usually started by prewetting the desired area with just water then paint is dropped in. By prewetting the paint spreads out on its own and is helped along with a brush. This is usually done for large washes.
You can also apply your paint directly to the surface. This is usually reserved for smaller shapes.
If you never learned any other painting techniques after this one you could still make some really interesting paintings.