On a piece of photographic film, such as the kind you use to shoot photographs, one side of the film is coated with a layer of chemicals called the emulsion. This is the side that absorbs the light, and the emulsion is scratch able and dull. The non-emulsion side of film looks shinier and is more difficult to scratch. You can see the emulsion side on any negative you have hanging around.
The kind of film that comes out of image setters or that a pressperson uses to print your brochure also has an emulsion side. If you are creating something on your computer that will be output onto film (rather than onto plain paper from your personal printer or onto resin-coated paper from the image setter), you need to know whether the film should be output emulsion side up or down. The only person who can tell you the correct answer is the pressperson who will be printing the final job. She will say, “I need your film right reading emulsion side up” (RREU) or maybe “right reading emulsion side down” (RRED). This means that if you were to lay the film on a light table as if you were reading it properly (reading it right) the emulsion would be up, or facing you, or it would be down, on the side of the film away from you.