Secure boot is a feature in the Unified Extensible Firmware Interface, the replacement for the motherboard firmware or BIOS. It has been implemented by Microsoft in a manner that effectively prevents easy booting of other operating systems on machines which have secure boot enabled.
An exchange of cryptographic keys takes place at boot-time so that a system can verify that the operating system attempting to boot is a genuine one, and not malware. There are further key exchanges along the way. Since Microsoft controls the key-signing authority, everyone who wishes to boot an operating system on hardware certified for Windows 8 has to buy a Microsoft key.
The fact that secure boot would be used in Windows 8 was known last September. The ideal solution would have been for all the Linux distributions, plus other companies that depend on Linux for their profits, to band together under the Linux Foundation and use their combined clout to influence things with hardware vendors.