I did not ask how to defend against it, I asked what it is called. But you and I both know you're intentionally 'misunderstanding' that.
usb keylogger dongle? firewire dma device? it's a whole class of crazy shit
attaching malicious devices to your computer
it's more "people fucking with the software on your hard drive"
full-disk encryption protects all the files, but the computer has to have something that tells it how to display the "decrypt drive" screen and then load the rest of the system. if somebody gets a hold of your shit somehow, and replaces this with a modified equivalent (i'm sure there are dozens of easy-to-use script-kiddie-friendly versions of grub that send off drive keys and let you put other payloads in the initramfs) that does malicious shit, then you're fucked. being fucked is generally a bad idea
And this is worth only being able to run microsoft approved software on a computer that requires hardware attestation with your ID via pluton to connect to the niggercattlenet. Champion. Really.
with a non-retarded implementation of secure boot, you can burn your own public key into a nigh-impossible-to-reset write-once non-volatile memory area, and run your own bootloader that you signed yourself
this means if you lose track of your computer for a while, and a moderately-sophisticated attacker replaces your bootloader with his special build of grub that inserts backdoors, it won't work because all of your hardware will say "fuck no this isn't
@SCV's grub wtf i'm not booting up now"
there are many attacks that somebody could do if they had access to your computer. the general situation is quite dire if somebody can touch your computer, which means ideally you should never ever let this happen. unfortunately nobody is perfect so there are tools available to potentially cuck attackers that are dedicated enough to get around a lack of security but not dedicated enough to get around moderate security
secure boot is but one technique in a wide arsenal that lets you lock down a computer to do 1 thing
it also gets a bad rap for being used by gay corporations to lock down computers to do 1 thing that isn't what the user wants. this is indefensible and any secure boot implementation that comes with somebody else's keys and no way to add your own is extremely gay and the person responsible needs to be subjected to medieval torture/execution methods