Backlit animation is an animation process used to create animation with light. By shooting animation mattes with light shining through them, filmmakers can create special effects that have a characteristic "glow" effect to them. This animation process was used extensively in TRON for the glowing circuitry on the bodies of the programs as well as the majority of the effects animation (i.e. sparks, lights on walls, background animation).
Backlit animation was pushed to its limits in TRON and made the animation for the movie very laborious.