Apple’s new-generation Macs come with a new so-called Apple T2 security chip that’s supposed to provide a secure enclave co-processor responsible for powering a series of security features, including Touch ID. At the same time, this security chip enables the secure boot feature on Apple’s computers, and by the looks of things, it’s also responsible for a series of new restrictions that Linux users aren’t going to like.
The issue seems to be that Apple has included security certificates for its own and Microsoft’s operating systems (to allow running Windows via Bootcamp), but not for the certificate that was provided for systems such as Linux. Disabling Secure Boot can overcome this, but also disables access to the machine’s internal storage, making installation of Linux impossible.
Which seems strange, considering most of the Apple computer growth seems to be Linux and Windows guys wanting to run on outdated Apple Hardware.