Secure Boot is a feature of UEFI, and it's a requirement for any computer that wants to run a modern version of Windows. It exists to protect us against malware that infects your computer's bootloader. There's a security certificate stored in the UEFI which your computer uses to check the Windows bootloader, to ensure it's legitimately signed by Microsoft, and not an imposter.

So far, so good, but what happens when the certificate in your UEFI expires? Well, we're all about to find out.