Processor microcode is akin to processor firmware. After a hardware release manufactures can post updates to their microcode, in order to fix stability or security concerns. As such, running outdated microcode can cause an unstable system – or worse pose actual security risks.
Usually the bios or UEFI is in charge of applying these updates on boot, but a lot of BIOS/UEFI ship with outdated microcode. Thankfully these can also be applied by the linux kernel with the following.
INTEL
sudo apt-get install intel-ucode
AMD
sudo apt-get install amd64-ucode
Once installed, trigger a grub update to ensure any microcode updates will be processed on boot.
sudo grub-mkconfig -o /boot/grub/grub.cfg