As such, there aren't many new features in this build; Microsoft's focus now is on fixing all the (obvious) nasty bugs so they can release the Fall Creators Update on 17th October as promised. However, we do get one new feature in this build, namely the ability to "Resume from phones to PC with Cortana", which essentially means that anything you had open in the Cortana app on your iPhone or Android phone, can now be "resumed" in the same state, at the same place, on your PC. (presuming you've paired your phone and PC already).
There's a raft of bug fixes as well, none of them desperately interesting, apart from the notable fact that this build actually installs (unlike 16288). Here's the full Microsoft release notes for the build.