Abbreviation for the word "duplicate". As a technical term, it means copying the original audio or video recording and making it double.
It is also used for the word "dubbing", which means recording over the original audio or video recording.
Dubbing is a post-production process where the original language of recording is exchanged with audio in a different language and then mixed with the audio of the media to make it sound as natural as possible.
The standard technique is one where the voice actors read the scripts out loud while watching the original film.
The dub is recorded by professional voice actors and the audio track of the original video footage is will be mixed with the alternate language recordings of the dialogue.
Whereas dubbing changes the original audio and sound, subtitling simply allows for the understanding the message without compromising the original audio.