The J-31 is a an experimental fighter aircraft being developed by the People's Liberation Army of China. The first known test flight was on October 30th, and a few pictures were leaked, probably with official permission. Since the aircraft's test flight was under a dozen minutes, and the pictures were taken from a distance, not much is known about the aircraft. It seems to be a light fighter, somewhat akin to the F-16 Falcoln or F-22 Raptor. Much like the Raptor, it seems to include stealth technology, which has led some to call it a "stealth fighter", although since nothing is known of its electronics or radar cross-section, this seems to be a guess. Also, since it is a lighter aircraft, there is some thought that it is meant to be part of China's plans for an aircraft carrier.
Since very little is known about the actual capabilities of the J-31, I would rather remark on what I have noticed in the media's reaction to it so far. China flew another "stealth" prototype, the J-20, last year, so the idea that China is developing two new first-class fighters seems to mean they are in the process of upgrading their air force. This could be taken as a sign of a desire for greater regional power, especially if the J-31 is able to deploy on an aircraft carrier.
Besides, of course, all we have now is a few pictures from a ten minute test flight. There is no evidence that this aircraft is designed for carrier flight, and despite some rumors to the contrary, China doesn't even have an operational aircraft carrier. If the J-31 is the equivalent of the F-22, it hardly means that China is drawing even (or a threat) to the United States' air capabilities, since the F-22 prototype first flew twenty two years ago.
As I wrote about the Chinese Aircraft Carrier program, much of what has been written in the West about the Chinese military is based on dichotomizing China as either backwards or a threat. An otherwise level-headed Atlantic article about the aircraft was entitled "China's Mysterious New Stealth Jet", and while I don't think it was intentional, the term "mysterious" certainly has connotations when talking about China. Perhaps the J-31 can transform into a tiger, and then destroy other planes just by chanting and concentrating on its chi?
The truth about the J-31 is that China is a powerful and advanced enough nation that it can now design and manufacture high quality, technologically advanced aircraft. But at this point, they are still a generation behind The United States and other NATO nations in being able to design and produce aircraft. The J-31, like most of China's military, and China itself, it developing greatly, but is not a present threat to the United States of America.
http://security.blogs.cnn.com/2012/11/02/new-pictures-show-second-chinese-stealth-fighter-being-test-flown/: A basic article showing pictures of the new aircraft.
http://www.theatlantic.com/international/archive/2012/11/everything-we-think-we-know-about-chinas-newest-stealth-jet/264425/: A skeptical look at where this aircraft fits in with China's larger military picture.
http://blogs.wsj.com/chinarealtime/2012/11/03/taking-off-implications-of-chinas-second-stealth-fighter-test-flight/: A somewhat giddy look at the aircraft, along with a giddy look at China's military ambitions.