Captions and subtitles are the dialog of a video in written text format. Sometimes they also include information to help folks follow the dialog, like descriptions of music, phones ringing, and other sounds in a video's audio track.
Captions and subtitles make videos accessible to a wider audience by allowing folks who can not otherwise understand the audio track to follow along, especially those who are hard-of-hearing or speak other languages. Captions are in the same language as the video's audio track. Subtitles are in a different language.
You can add captions/subtitles to videos you've uploaded. It's optional but strongly encouraged to make your videos available to as many people as possible.
