Definition:
Filming gets the picture. It does not always get the sound. ADR is the post-production process where actors return to a studio, watch their performance on screen, and re-record their dialogue in sync with what they already shot. The set recording gets replaced. The studio recording takes its place.
What It Means in Movies:
On location, sound is at the mercy of everything: passing traffic, wind, a generator humming two rooms over. ADR is how filmmakers fix that after the fact. The actor watches the scene play back and matches their new recording to the lip movements they can see. When it is done well, nobody notices. When it is done badly, the dialogue feels slightly disconnected from the body delivering it.
Why It Matters:
Without ADR, a significant portion of location-shot dialogue would be unusable. Outdoor action sequences, crowd scenes, anything shot near airports or busy roads: all of it depends on ADR to arrive clean in the final mix. It is also used creatively. Directors sometimes use ADR sessions to push a performance further, or to adjust a line reading months after the shoot wrapped.
Example:
A chase sequence shot through a crowded market produces dialogue that is buried under ambient noise. In post, the actors come into a sound studio, watch the scene, and re-record every sync line. The original location audio is stripped out. The ADR takes its place in the mix.