Unified Extensible Firmware Interface (UEFI) is firmware code from a chip on your motherboard that provides extra functionality, beyond the Basic Input/Output System (BIOS). UEFI is a way to do things with your computer before an operating system is loaded. To name a few features, UEFI can provide extra graphic menus and system information on a computer, and basic diagnostics on both a computer and a tablet. How does it do this? UEFI works with the BIOS to initialize the computer or tablet's hardware before handing hardware control over to the operating system. UEFI can also load and display software programs from storage devices to provide even more functionality, such as more detailed system information and extended hardware diagnostics.
Figure : Concept drawing of boot/startup flow showing UEFI