The UEFI is a development by Intel and Microsoft that supports what is called Secure Boot, which requires all the loaded firmware to be signed or it won't be loaded. This is a problem for Linux, since the keys required for signing must, under the current GPL, be made public. This, of course, defeats the purpose. There are several workarounds, including some being used by Red Hat, SuSE, and Ubuntu, which are being discussed by the Debian developers and will probably be included in an update at some future point.