Every traditional computer contains a BIOS. The word BIOS stands for BASIC INPUT OUTPUT SYSTEM. The BIOS is a small peace of firmware (built in software) that controls the hardware components inside your computer.
When a computer is turned on, it initializes the hardware components and runs the POST (Power On Self Test). Once the power on self test completes, and depending how the BIOS is configured, it would look for a bootloader program. The bootloader, then loads your operating system like Windows.
You can enter the BIOS and change various settings by using its user interface. When the computer first starts the BIOS can be accessed by pressing the delete key or one of the function keys, generally DEL,F2 or F10. When in the user interface you can change settings like the date and time, disable built in hardware or change the boot order.