The content is "an array of mini-dungeons" called fractals, which are in fact "slices of the Mists," which themselves are fragments of time. It's not as confusing as it sounds at first; think of each fragment as an historical vignette, some of which may be familiar to you if you're a Guild Wars veteran.
ANet has made the dungeon accessible to everyone via a side-kicking system, though the dev diary does say that the aim was to challenge level 80s. Not only that, but every time you revisit Fractals of the Mists, it scales up in difficulty. Take a look at a preview clip after the cut.