What is UEFI?

UEFI is designed to be much more flexible than the old PC BIOS that dates back to some of the first IBM PC’s since 1979. UEFI stands for Unified Extensible Firmware Interface. The BIOS (Basic Input Output System) is installed directly onto your computer's motherboard and is basically a bridge between your hardware and the operating system.

The BIOS runs after you turn on your computer, but before the operating system boots. It tells the operating system what hardware you're using. The BIOS has not changed much in the past 25 years and is one of the main reasons why a PC’s boot-up time is over 25 seconds.
UEFI is very different from old BIOS:

Experts expect UEFI to start gaining a significant foothold in the computing market from as early as 2012 next year. Many consumer electronics companies are working hard to reduce the boot times of their machines, a factor that is particularly important with mobile devices and tablet computers.