The BIOS performs a power-on self-test (POST) (a built-in diagnostic program that checks system hardware to ensure that everything is present and functioning properly, before the BIOS begins the actual boot), a test which is used to ensure a system is functioning properly. When a problem is identified, the BIOS will normally produce an error message. In some cases, since a problem may be detected so early that the BIOS cannot even access the video card to print the message, a series of beeping pattern will be produced on the speaker to tell a user what the problem is. The exact meaning of the beep codes depends on the type and version of BIOS. A site to check for BIOS Post Codes is "BIOS Central" -- please note the box upper right-hand side identifiying the system BIOS used on the system -- this you must know.
Note: A single beep during the boot process -- usually right before the BIOS startup screen is displayed -- is normal and does not indicate a failure as long as the boot process continues.