Screen printing is a process that involves pushing ink through a mesh screen onto a substrate.  The mesh screen is made into a stencil with your design and ink is pushed through with a squeegee.  One screen is required for each color in the design being printed.  Designs with multiple colors require each color to be separated on its own screen.  One color screen prints are cheapest due to only needing to prepare one screen for printing. 

The following is an example of how each color is separated and stenciled onto the screen for a five color design.  We printed this on white t-shirts so there was no need to separate the white in the clouds/eagle, we just used the t-shirt color.  Each color is screen printed one at a time, generally from lightest color to darkest color in the design.