Introduction to Automated Dialogue Replacement.
If you’ve never heard (heh) of ADR, here’s an example:
A speedy, yet comprehensive, overview of ADR for Beginners (via SBN3):
Filmmaker IQ offers more context for the practice of adding dialogue in post-production:
Try It Yourself
Shutterstock‘s handy tutorial on How to Record ADR Dialogue:
Whether your dialogue is recorded during production or post-production, it will need post-processing – here are the need-to-know techniques: