Unified Extensible Firmware Interface, or UEFI, is firmware installed on a motherboard and starts when you turn on the PC. This special software loads before the operating system and checks if all components are connected and working properly before letting the OS take over.
As opposed to BIOS (basic input/output system), which has limitations in how many commands it can run concurrently, UEFI is more flexible, faster, and easier to use as it looks like a slim version of an OS. Most PC systems today use UEFI instead of BIOS.