Just what is ADR? ADR is Automatic/Alternative Dialogue Replacement. ADR is used when a production’s on-set soundtrack of the dialogue can’t or won’t be used. It is also known as Looping or Dubbing.
According to IMDB:
The re-recording of dialogue by actors in a sound studio during post-production, usually performed to playback of edited picture in order to match lip movements on screen. ADR is frequently used to replace production track of poor quality (e.g., due to high levels of background noise) or to change the delivery or inflection of a line. ADR can also be used to insert new lines of dialogue which are conceived during editing, although such lines can only be placed against picture in which the face of the actor speaking is not visible.
Here is a behind the scenes video from King Kong’s (2005) ADR session: