ADR stand for "Automated" or "Automatic" Dialog Replacement.
Dialog that is flawed on the production tracks must be re-recorded in a process called looping or ADR.
Looping originally involved recording an actor who spoke lines in sync as "loops" of the film were played over and over. ADR is painstaking work. Some actors are much better at it than others.
An actor watches the image repeatedly while listening to the original production track on headphones as a guide. The actor then re-performs each line to match the wording and lip movements. Actors vary in their ability to achieve sync and to recapture the emotional tone of their performance. The director needs to supervise and direct the ADR sessions.
ADR is usually considered a necessary evil but there are moments when looping can be used not just for technical reasons but to improve the performance or interpretation of a scene or the even change a few words of dialog.
Every attempt should be made during production to get the cleanest dialog possible in the hopes that some or all of the film will not require ADR.