Photosensitive emulsions are applied to screens and then let dry. You then take a black image on transparency or translucent surface, place it against the screen, and then expose the screen to light. The light causes the emulsion to harden and bind to the fabric.
Where the light is exposed to the screen, the emulsion binds, making a solid layer. Where the light is blocked from exposure to the ligh the emulsion remains water-soluble. After exposiure, the screen, is sprayed with a high pressure washer, washing off the unexposed and water-soluable emulsion where your image was placed; this clear area is where ink will be pressed through the screen when you print.