Those of you with long memories will recall a barrage of complaints in the run up to Windows 8's launch that concerned the ability to install other operating systems—whether they be older versions of Windows, or alternatives such as Linux or FreeBSD—on hardware that sported a "Designed for Windows 8" logo.
To get that logo, hardware manufacturers had to fulfil a range of requirements for the systems they built, and one of those requirements had people worried. Windows 8 required machines to support a feature called UEFI Secure Boot. Secure Boot protects against malware that interferes with the boot process in order to inject itself into the operating system at a low level. When Secure Boot is enabled, the core components used to boot the machine must have correct cryptographic signatures, and the UEFI firmware verifies this before it lets the machine start. If any files have been tampered with, breaking their signature, the system won't boot.
This is a desirable security feature, but it has an issue for alternative operating systems: if, for example, you prefer to compile your own operating system, your boot files won't include a signature that Secure Boot will recognize and authorize, and so you won't be able to boot your PC.