ADR stands for “Additional Dialogue Recording”, the term used to describe the process of re-recording actors’ dialogue in a controlled environment (i.e. a sound studio) during post-production. During the ADR process, actors are called back to the studio to lip-sync lines recording during principle photography in time with a projection of scenes and/or a rough cut of the film. The main reason for re-recording lines is to obtain clean sound, however ADR is also sometimes used to subtly adjust an actor’s performance in a scene (by having them deliver the same line with slightly different inflection). It can also be used to add new non-sync dialogue to aid the story (i.e. record a line which is then “delivered” while the actor has their back to camera).
ADR is used extensively on big budget films, since it is almost impossible to get clean sound on a set where several hundred people may be working, but is also extremely valuable for independent films where you don’t have full control of the location (and therefore may end up with unwanted background noise in your sound).