What is secure boot ?
Secure boot is a hardware feature that allows only signed/verified OSes to boot on the machine. In its benign form, secure boot is meant to protect the PC against boot loader attacks, which can compromise a system before the OS even loads.
However, the FSF warns that this feature could be used to curtail the freedom of computer users by restricting them from installing alternate operating systems such as Linux on their machine. In other words, you can say goodbye to dual booting multiple OSes on your machine.
Why is this important ?
Because Microsoft's Windows Certification program will require that all certified Windows 8 systems have secure boot enabled by default. The UEFI secure boot protocol permits one or more signing keys to be installed into a system firmware.
The problem for Linux is that it won't have any such signature by default, meaning you won't be able to install it on a Windows 8 certified machine.
So to spread awareness about this, the FSF is calling all people to join the signature campaign and stand up for your Freedom.