BIOS (Basic Input/Output System) is the firmware used to initialize and manage hardware components during the startup process of a computer. It performs a POST (Power-On Self-Test) to check if the hardware is functioning correctly, and then it loads the bootloader or operating system from the active partition on the storage device. BIOS is stored on a chip on the motherboard and provides a low-level interface between the operating system and the hardware. Users can access the BIOS setup utility to configure hardware settings, such as boot order, system time, and peripheral device settings. While BIOS has been largely replaced by UEFI in modern systems, it remains an essential part of the boot process in many older computers.