A couple of years ago both Intel and AMD started developing their own solutions that allows them to offload some the functionality that used to happen on the main processor to another co-processor placed inside the chipset on the motherboard, functionalities like power management, secure booting, USB handling..etc

The problem is not only that this co-processor is running it's own operating system with full unrestricted access to the systems main memory and network interface, but also that the main processor can't actually tell what the co-processor is doing with this data or when does it access it, on top of that both Intel and AMD delivers you the software running on the co-processor as a binary blob so we can't really be sure if it's secure, has backdoors, spying on you, etc..

LibreBoot/Coreboot are projects to write an open-source BIOS for x86 hardware, and since a big part of the BIOS is running on this co-processor you only have a choice of either using the binary blob provided by Intel/AMD or lose the functionality provided.