It's an in-development feature to block parts of websites that track you. In practice, it mostly blocks ads, and it blocks most ads. And it doesn't have the performance/memory hit of AdBlock Plus for two reasons:
* The domain matching is simpler (no regexps) and thus much faster.
* It doesn't use CSS to fix up the layout of pages that have had ads removed; it just leaves blank white rectangles. (AdBlock Plus has lots of complex and even site-specific CSS code to do this.)
(I've been told this by one of the Tracking Protection developers, though I may have got the details slightly wrong.)
In testing done by its developers, it speeds up the load time of the median page by 20%! So overall it gives more privacy, less page clutter, and faster browsing. Fantastic stuff.
What's funny is when you take the time to ensure than your layout will reflow nicely with ad blockers.. actually having the empty spaces is a bit worse imho.
Wondering about catching on-error events for iframes.. or load errors, etc... that may help in detection, and reflow. I'm more concerned about things continuing to look nice with adblockers than circumventing them.
<:D