UEFI (Unified Extensible Firmware Interface), also called UEFI BIOS, is the most common form of one of two different types of motherboard firmware (the other being BIOS). Firmware is software embedded into hardware. Hardware like GPUs, hard drives, SSDs (solid state drives) and more can also come with firmware. Motherboard firmware determines everything from the boot drive to the CPU clock speed to which components are enabled.
While the old legacy BIOSes couldn't handle large storage drives, UEFI can handle drives that are 2.2TB or larger. UEFI also has much richer setup menus than legacy BIOSes.
This article is part of the Tom's Hardware Glossary.