See that green bugdroid above? It's carrying a heavy(ly customized) version of Marshmallow specially to your Huawei Honor 5X. Wrapped tightly with it is a new version of EMUI for the device: 4.0.
In terms of Marshmallow, you should expect the usual features that we've been talking about for nearly a year now: Now on Tap (which is getting more useful with time), Doze mode for better battery optimization while idle, better volume controls, a new open apps menu, and more.
As for EMUI 4.0, it brings a few interesting modifications. There's motion control to launch different apps or perform some actions while the screen is off (draw M to launch the music player, double tap to wake, etc), a personalized navigation bar with a notification drop-down shortcut à la LG, one-handed mode, different drag locations to drop the notification shade or the toggle shortcuts, and a few more.
The update should be live for the device now or at least it started rolling out already. You can head over to Settings > Update to check for the OTA and start downloading it.