To make a screen, the printer coats the mesh with emulsion, lets it dry in the dark, then places a film positive of one color of the design against it and exposes it to UV light. Light hardens the coating everywhere except under the black areas of the film. A rinse with water clears out that soft emulsion, leaving open mesh in the exact shape of the art.
The type of emulsion matters. Water-based and discharge inks soften stencils that are not water-resistant, so printers use dual-cure or other water-resistant emulsions for those jobs. Thickness matters too: a thicker coat, or a capillary film, builds a taller stencil that lays down more ink. That is how high-density prints get their sharp, raised edges.
Poor exposure shows up as pinholes, ragged edges or a stencil that breaks down mid-run. It is a workshop detail, but it explains why each color needs its own screen and why setup is a real part of screen printing cost.