Meaning of "synchronized media"

Hello --
I have a question about the meaning of "synchronized media", because the
terms used seem very specific. This is related to SC 1.2.5 in WCAG 2.0:
https://www.w3.org/TR/UNDERSTANDING-WCAG20/media-equiv-audio-desc-only.html

It defines synchronized media as:
"audio or video synchronized with another format for presenting information
and/or with time-based interactive components, unless the media is a media
alternative for text that is clearly labeled as such"

My confusion is around "audio or video synchronized with another format" --
what is "another format"? What would some examples of synchronized media
be? Does this necessarily include something like a movie?

So does this mean that all videos (I mean, audio & video, like what you
would see on youtube) would require described audio in order to meet this
SC?

Thanks!
-Robin Hillman