A BIOS is special low‑level software built into a computer’s motherboard that runs first when the machine is powered on. It checks the hardware, then starts loading the operating system so the computer can boot.

Basic meaning

• BIOS stands for “Basic Input/Output System” and lives on a small memory chip on the motherboard as firmware.
• It acts as a bridge between the hardware (CPU, RAM, storage, keyboard, etc.) and the operating system during startup.

What BIOS does

• When the computer turns on, the BIOS runs a hardware check called a Power-On Self-Test (POST) to make sure components are working.
• After that, it looks for a boot device (like your SSD or hard drive) and hands control over to the operating system so it can load.