An AA (Anti-Alias) filter is a filter that is placed in front of a sensor in a camera. It is also known as OLPF (Optical Low Pass Filter). It typically is used in all DSLR and MirrorLess cameras (and may be used in other formats). It attempts to control the Moire effect as seen in the video above. The filter effectively blurs the images slightly to reduce the moire effect.
Recently some manufacturers have been adjusting the amount of blurring caused by the AA filter or have been eliminating it all together to increase resolution. The first that I heard of was Olympus back 3 years ago with the introduction of the E-5. They basically took their older sensor technology and dramatically reduced the AA filter in the new E-5. DPReviews (DPReivew.com) initial reaction was that Olympus was taking a big chance and might very well pay a serious price by reducing overall Image Quality (IQ). This turned out to be an over reaction as the E-5 did quite well, even with their dated sensor technology.
The next manufacturer to tweak their AA filter was Nikon on the recently released D800E. They didn't eliminate it but place a final filter behind the AA but before the sensor that effectively canceled out the AA filter's blurring effect. It is not understood by anyone, that I've read, why they did this. DPReivew speculated that it might have had something to do with the manufacturing process, making it easier to create both the D800 (Standard AA Filter model) and the D800E (Canceled AA Filter model) to be created side by side on the assembly line. Whether this is true or whether Nikon is experimenting for some future use we will not know but one thing became clear that this did not cause any significant problems in IQ. Quite to the contrary the D800E has been very well received. I've not heard of any significant problems of moire being produced by the D800E, certainly nothing that could not be taken care of in post processing.