It is basically your mobos operating system. Through it all the parts work/comunicate with each other. It stands for Basic Input/Output (System). Originally computers had no hard drives to have an operating system run it.
To expand upon this; The BIOS reads and initializes all of your systems devices and gives you an essential bit of control over your system. The BIOS is static, meaning it never changes and should always work unless your hardware fails. Which is why you should be extra careful when flashing/updating a BIOS. The most important role it plays after all of this is to boot up your operating system. The BIOS can boot an OS from any number of devices, HDDs, optical drives, floppies, USB drives. From that point on the OS takes over and you don't really see or interact with your BIOS.