Its a coating on the back of 35mm film made for cinema work. It protects the film base from being scratched, an issue with movie film because it moves at such high speeds through the camera (usually 24 frames per second). It also, I think, serves as an anti-halation layer. Movie films are c-41 process, but if you run them through a normal c-41 process, the remjet comes off and contaminates the chemicals. Cinestill removes it from the unexposed film so it can be used by still photographers and processed in normal photo labs.