Photo Emulsion is a vital part of a silk screener's arsenal. It is a photosensitive, thick liquid that, when combined with the sensitizer, is used as a medium to "burn" an image onto a screen.
The long and the short of it is that the photo emulsion and sensitizer mixture can be applied to a silk screen and, after it has dried, an acetate sheet with an image printed on it can be placed on the dried emulsion mixture then exposed to light. The area of your image that is dark is the area that ink will go through when you're ready to print.
After about a minute remove the acetate sheet and rinse the screen to open up the unexposed sections.