About XNU Kernel
XNU kernel is part of the Darwin operating system for use in OS X and iOS operating systems. XNU is an acronym for XNU is Not Unix. XNU is a hybrid kernel combining the Mach kernel developed at Carnegie Mellon University with components from FreeBSD and C++ API for writing drivers called IOKit. XNU runs on I386, X86_64 for both single processor and multi-processor configurations.
XNU used in OSX, and has always been open source for about 20 years. This is them updating to release the support for ARM too, which means iOS. The kernel however is just one tiny spec of iOS. By giving access to the kernel code, Apple has made it easier for developers to understand how devices and upper layers of software work with the kernel. It is not yet clear how much more the developers can benefit from this release, but it could be one of many steps taken by the company to attract developers.
The source code is supplied with the Apple Public Source License 2.0, which is a rather restrictive license; developers might want to check the details before planning to include parts of the newly released kernel code in their projects. You can find more information about building and testing the XNU kernel on its GitHub repository.