Not sure how new, and apologies if I'm lagging behind, but I do check the sources on Wikipedia all the time (no really, I do), and this is the first time I've noticed this feature. If it's not clear from the screen shot (even after clicking to enlarge), what happens is this: If you hover your mouse pointer over a link to a reference (a footnote number), a balloon tip sort of window will appear, and the text within will be what's found at that link. (Try it on the actual Wikipedia page.) Saves scrolling down to the bottom of the page and back. Even better, if the reference note contains a link to the source, as they often do, that link will be displayed in the balloon tip window, ready for right-clicking and opening in a new tab. Very handy!
Thanks, Wikipedians. Or MediaWiki people. Or both.