It's their very low-end 7" tablet which they're selling for only $50. And no, that is not a typo.
I haven't seen any reviews, or even much information about the specs, so it's hard to comment much. What I did see is that the processor, of course, won't be as powerful, and the screen won't be as high resolution. But for what is almost literally a "throw-away" device, I suspect it will be fine for people who want something very basic that works solidly, just not as crisply as better models. Also, I believe that the screensaver will show ads, though you can get rid of that for $15 more. Further, Amazon is assuming that families might want to get these tablets for everyone in the family to eliminate sharing, or so they can be left around the house, and selling a "six-pack" for the price of five, which drops to price to about $42 a unit. It'll even be sold like a six-pack.
Also, speaking of families, Amazon is doing something interesting, selling a kids-friendly version of the tablet. It will cost more, $100, but apparently will be a bit more protectively made to guard against being dropped, and also the browser will be configured so that only pre-approved age-appropriate websites (about 20,000 of them) and YouTube videos can be accessed.
Early this year I wrote about HP selling their new Stream line of 7" Windows tablets that sell for only $100, This crushes even that. (I've been using one for the year, and it's terrific -- and to be fair in comparing prices, running Windows as it does, it's a real, full computer on which you can do serious work, and also it includes a free one-year subscription to Office 365, which is a $99 value, plus 1 terrabyte of OneDrive cloud storage for free, another significant added value. So, depending on your needs, it's the equivalent of free -- or even getting money back. I don't mention this for comparison purposes, but for the sake of accuracy.)
The reason Amazon is offering such an incredibly low price is because they are taking on Apple. Apple devices are high end and cost a lot, which is where Apple makes its money. Amazon, however, wants you to buy its services, like Amazon Prime, and that's where they make their money.
If you want to read a little bit more on all this -- details on the device and everything else that Amazon announced are sketchy -- you can read it here.