A line in the code reads "Unlimited free storage for original quality photos - videos uploaded from your Nexus device."
Currently, Google Photos caps photo size at 16 megapixels and videos at Full HD 1,920x1,080-pixel resolution. The introduction of new feature means that the photo sizes and video resolution of Nexus owners will no longer be capped, and photos and videos can be backed up and stored at their original size.
It is not a big deal for photos, since the best Nexus camera is capable for shooting 12.3MP, but it makes room for higher megapixel cameras in the future. However, the Nexus 6P and 5X can shoot video at 4K UHD 3,840*2,160-pixel resolution.