Basically, if your book is in KDP Select, and has been in it for 30 days or more without a price change, and the price is between $2.99 and $24.99, you can put the book on sale for up to a week; you can even have it go on deep discount to start and then incrementally go back up during the week.
Of course, any KDP author can change the price of their book any time, but this new feature has some real advantages. The biggest is that Amazon customers see the original price and the sale price and know it's currently discounted and how long that discount will last. The second is that if the “real” price of the book is eligible for the 70% royalty rate (which means it has to be at least $2.99 but not more than $9.99 in the US store), then the author keeps that rate even when the price drops during the promotion. So if your book is on sale for 99¢, you still get 70% of 99¢ instead of 30% (i.e., 69¢ per books instead of 30¢). It looks like right now Countdown Deals might be available only in the US and UK Kindle stores because those are the only stores for which price ranges are listed, but I don't know if that's certain.
There's also a special website for book browsers to see what's on sale, and there's a new Promotions link on the KDP Reports page to show more elaborate reporting of Countdown sales, letting you see how each price point is doing.
I don't have any KDP Select books at the moment, but once I launch Saronna's Gift, I plan to use this new feature.
The KDP Reports page also added a way to search the current month's reports for specific books, which I don't find terribly useful since it seems to apply only to the selected Kindle store. Now if it searched for a specific book across all the Kindle stores, that would be helpful. There's a View All link but it doesn't seem to look at all the stores, either so I don't see it as helpful at all.
All in all, this is a positive development. It would be mush more useful if it weren't limited to books in KDP Select, but Amazon's commitment to authors extends only as far as they think it will make money for them. It's important to remember that.