This marks a huge step forward for Apple, a company not exactly renowned for embracing open source. Cupertino likes to keep its software under wraps, where it believes it can keep complete control of its output and users.
However, Apple has increasingly said in recent years that it is embracing open source. That has not exactly been evidenced so far, but the release of the XNU kernel is a major step forward.
This is big news because it means even Apple is starting to understand the power of an open source tech community.
Apple's XNU is short for ‘XNU is Not Unix' and it is, ahem, a Unix-like kernel. Apple uses it one Mac OS versions, such as OS X, while it is also the underpinning system for iOS. The company has rolled out the software on GitHub.
By opening access for the system, Apple is opening the door for developers to understand how their own software interacts with the kernel. It is not a huge ability for sure, but again it at least opens the door to Cupertino embracing open source.
The source code comes with Apple Public Source License 2.0. This is a limited license, so most developers are unlikely to get much from the kernel. However, you can find more information by heading to the XNU kernel on GitHub.