Post Sync ADR (automated dialogue replacement) is dialogue, which is added in post production to replace or supplement the O-sound, i.e. dialog recorded on set.

This may be necessary for different reasons. Often, the technical quality of the O-sound is not good enough, because it was shot on a loud, busy road, yet the scene is set to play in a quiet environment, and the road is not in the frame.

Other reasons include changes to the script, additional dialog, or miscued or bad dialog.