BIOS (Basic Input/Output System) is the small firmware that runs the instant you press the power button, before Windows even loads. Its job is to check that the key parts are present and working, then hand control over to the operating system.
What it does
- Power-on self-test: checks the CPU, memory, and drives are present.
- Boot: finds the drive that holds Windows and starts it.
- Setup: holds basic settings — boot order, date/time, which devices are enabled.
Why you'd see it
You normally never touch it. But if the PC can't find Windows, or a part fails the self-test, it stops and shows a screen — often "Press F2 or DEL to enter Setup" — that's the BIOS telling you something needs attention.
BIOS vs UEFI
Newer PCs use UEFI instead of the old BIOS. UEFI is faster, supports larger drives, and gives a more modern setup screen. Most people just think of both as "the thing that boots the PC."
If you're stuck on a "press F2 to enter setup" loop, the PC is failing to find a boot drive — usually a loose or failing disk, or a changed boot order. That's a job worth a technician if it keeps happening.