BIOS stands for Basic Input/Output System, and it is a firmware that is built into a computer's motherboard. BIOS provides a fundamental set of instructions and settings that enable the computer's hardware to function properly.

The BIOS is responsible for performing a series of tests, called the Power-On Self Test (POST), which verifies that the computer's hardware is working correctly. The POST also checks for the presence of essential components such as the CPU, memory, and hard drive, and ensures that they are properly connected and configured.

Once the POST is complete, the BIOS loads the operating system into the computer's memory, allowing the user to interact with the computer. In addition, the BIOS provides an interface for accessing and configuring various hardware settings, such as the boot order, CPU clock speed, and memory timings.

While the BIOS has been the primary firmware used in computers for many years, it is gradually being replaced by newer firmware technologies such as UEFI (Unified Extensible Firmware Interface). However, BIOS is still in use in many legacy systems and remains an essential component of modern computers.