BIOS and UEFI are motherboard startup firmware. They run before the operating system, initialize hardware such as memory, storage, and peripherals, and hand control to the bootloader. BIOS is the older model; UEFI is the modern replacement, with more features such as Secure Boot, better hardware support, and a richer recovery environment.
This firmware matters because it sits below normal security tools. If it is misconfigured or vulnerable, attackers may change boot settings, weaken Secure Boot, or gain a durable foothold that survives an OS reinstall. Defenders use vendor updates to fix firmware bugs, and they should verify the exact motherboard model and version before flashing. In practice, BIOS/UEFI issues appear in advisories for critical motherboard flaws, in bootkit attacks, and in defensive checks for firmware integrity, update hygiene, and recovery planning for encrypted systems.