Simply put, a silkscreen is printing technique where typically a series of stencils are placed on a wire mesh one-by-one with each stencil having a different colour ink applied. The ink is pressed through the mesh using a “squeegee”. The resulting ink/images that appear on the paper overlap/fit together to form the resulting image. Silkscreens can be made using, for example, 30 individually applied screens.
Here are a few examples: