ADR (Automatic Dialogue Replacement)   

Also known as looping, ADR describes the process of replacing dialogue which is not usable in the final mix or adding new dialogue. (required because of scenes being cut during post production or characters were off screen, etc.)
Originally done by projecting a loop of film to enable the actor to rehearse.  Film loop might have a streamer or a punch to provide a visual cue for the performer.  Some electronic systems also provide a series of beeps as an aural cue. Difficulties in matching performance and recreating original ambiance/perspective make this a tricky business, even with digital tools.