This one's for you developer-types. Google has just pushed the Android 5.0 kernel sources for the Nexus 9 and Nexus Player. Head over to the Android Open Source Project (AOSP) and poke around at your leisure. For non developers: a thing happened that is good, but it's not something you personally need to worry your pretty little head over.
Kernel source gives developers the tools they need to make ROMs and troubleshoot app incompatibilities on new devices. The kernel is the underlying software that makes the part of the platform you interact with functional and able to talk to the hardware. Android is open source, so every device maker has to eventually get the source code out and make it available for download.
If you want to dig into the code for these first Android Lollipop devices, the Nexus 9 is in AOSP at android-tegra-flounder-3.10-lollipop-release, and the Nexus Player can be found at android-x86_64-fugu-3.10-lollipop-release.