Ultra HD is marketing-speak for 4K or 8K resolution TVs. And that's more or less it. You're going to be hearing the term a lot this show though—and throughout the year—so here's a little more information about what exactly that means.
As you might already know, the industry measures a TV's resolution by its vertical pixel count. So a 1920x1080 TV is "1080p". But for 4K and 8K resolutions, we've for some reason switched to the horizontal measurement. For example, the "4K" resolution is 3840×2160. The official name for that resolution is Quad Full HD (QFHD), but 4K has stuck. Likewise, the 8K resolution (which, confusingly, is officially called Ultra HD or UHD) is 7680×4320.
All of that you might or might not have known. But announcements for new TVs are mostly using Ultra HD to mean 4K, or, sometimes, 8K. Maybe the most important stat, though, is the amount of content available for these hyper-pixeled displays: next to none. Now you know.