Can someone ELI5 this?
Over time, these instructions have gotten more and more complicated. Now there are "instructions" like "Enter Virtual Machine Monitor" which actually complex manipulations of tons of different registers, memory translations, and subsystems inside of the CPU.
And, even simple, primitive instructions like call, jump, and return actually need to check the state of various pieces of the processor and edit lots of internal registers, especially when we start to consider branch prediction and issues like Spectre.
It wouldn't be very plausible to hard-wire all of these complex behaviors into the CPU's silicon, so instead, most instructions are implemented as meta-instructions, using "microcode." "Microcode" is just software that runs on the CPU itself and interprets instructions, breaking them down into simpler components or adding additional behaviors. Most CPUs are really emulators - microcode interprets a higher level set of instructions into a lower level set of instructions.
Historically, Intel and more recently AMD have encrypted this "microcode," treating it as a trade secret. This makes people who are worried about secret hidden backdoors _very_ worried, because their CPU's behavior is depending on running code which they can't analyze or understand. This has led to all sorts of mostly unfounded speculation about secret back doors, CPUs changing the behavior of critical encryption algorithms, and so on and so forth.
Decrypting this microcode will theoretically allow very skilled engineers to audit this functionality, understand the implementation of low-level CPU behaviors, and find bugs and back-doors in the CPU's interpretation of its own instructions.
Replacing this microcode silently would be absolutely catastrophic security-wise, because an attacker could silently change the way the CPU worked, right out from under running software. But, there is no evidence this is possible, as the microcode is digitally signed and the digital signature implementation, so far, seems to be correct.