BIOS stands for Basic Input/Output System. It's a small program stored on a chip on your motherboard that runs before Windows, macOS, or Linux boots up. Think of it as the computer's first brain: it checks that all your hardware is working, then hands control to your operating system.
When you switch on your PC, the BIOS performs a quick health check on your RAM, storage drives, and other components. It also sets the order in which the computer tries to boot (for example, checking the hard drive first, then a USB stick). This is useful if you want to install a fresh copy of Windows from a USB drive.
Most users never need to open the BIOS. You enter it by pressing a key during startup, usually Delete, F2, or F10, depending on your motherboard maker. Inside, you can adjust settings like the time, boot order, or enable/disable certain ports. Some enthusiasts tweak CPU voltage or memory timings, but this is risky and rarely worthwhile for ordinary laptops or desktops.
Older BIOS menus were text-based and confusing. Newer motherboards use UEFI, a more modern version with a graphical interface and better security features. You'll see both terms used interchangeably, though UEFI is now the standard.
When buying a computer, you don't choose a BIOS: it comes with the motherboard. You might notice manufacturers highlight BIOS update support, which means they'll release new firmware to fix bugs or improve stability. That's a good sign of long-term support, but it's not a key buying factor for most shoppers.