Okay. So it brings a lot more than just responsive support to things like YouTube videos, but that is definitely one of the highlights!
No longer will you be required to use things such as FitVids.js (an awesome script, by the way) in your Bootstrap projects just to make videos responsive. And if you did it manually, well, you don’t necessarily have to anymore! (unless that’s your thing)
Check out the docs for more info.
And for more information on this awesome update, view the Bootstrap Blog.