Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink as a sharp-edged image onto a substrate. A roller or squeegee is moved across the screen stencil, forcing or pumping ink past the threads of the woven mesh in the open areas.
Screen printing is also a stencil method of print making in which a design is imposed on a screen of silk or other fine mesh, with blank areas coated with an impermeable substance, and ink is forced through the mesh onto the printing surface. It is also known as "silk screening" or "serigraphy".
The follows is diagram of how the basic screen printing works, it is only a sample of one color printing, as to more color, you have to repeat the same step for each color, that’s why screen printing was only for bulk quantities.