Computer users of a certain age will remember BIOS as ubiquitous firmware that came loaded on PCs. It was the thing you saw briefly before your operating system loaded, and you could dig into the settings to change your computer’s boot order, enable or disable some features, and more.
Most modern PCs ship with UEFI instead. But most also still have a “legacy BIOS” mode that allows you to use software or hardware that might not be fully compatible with UEFI.
In a few years that might not be an option anymore: Intel has announced plans to end support for legacy BIOS compatibility by 2020.
This most certainly affects many older operating systems – especially older hobby and alternative operating systems that were never updated with UEFI support.
This should only be a serious issue for 16-bit OSes or for bootloaders, as using multiboot (or more generally any bootloader that puts the machine into a known state regardless of UEFI/BIOS) the differences between UEFI and BIOS were largely abstracted from 32-bit and 64-bit OSes.