Unfortunately at the moment due to the lack of a changelog, it is unclear as to what kind of changes Google has made to Android 5.1, but it is safe to assume that it will come with the usual bug fixes and improvements, and as Android Police have pointed out, hopefully a resolution to the memory bug problems that have resulted in slow performance.
Android Police have also pointed to their server logs that show that a variety of different devices running on Android 5.1 have stopped by their website, although these devices are those of the Nexus variety which explains why they might have gotten the update first and might be used by Google employees.
Given that Android 5.1 is a big jump over previous Android builds, we’re hoping that Google will bring about some interesting new features and functionality. No word on when Google will be pushing the update out, but considering that it has already made its way onto Android One devices, hopefully it won’t be too long.