It is nothing less than all 257 measures of Bach's Chaconne in D Minor, which might just be the greatest piece of music ever written. And it's all there on one page. If you click on that image above, you'll be able to see a much higher-res version, where are all the notes are actually pretty clear if you zoom in closely enough. Or, you can download the same thing in PDF format.
I have much more to say about this kind of thing, but for now I'll just say that I find it quite beautiful as a sort of snapshot representation of this:
Happy Birthday, JSB!
See also: Looking Bach (from earlier today)