This is a non-FreeBSD specific term. It features in this wiki for reference.
The BIOS, short for Basic Input/Output System, is a program stored in a special chip on almost all motherboards especially i386 based systems. It is the first thing the CPU executes and sets the computer into a stable state with which to begin loading an operating system.
Some BIOS programs permit the user to change the settings affecting the way the system will boot, typically in which order to boot the physical drives. A typical example would be to set the CD-ROM (or DVD-ROM) drive to boot before the hard drive in order to setup an operating system, and then change it back following the installation.
Early IBM compatible PC-based operating systems, such as DOS and initial versions of Windows, used the BIOS to access the physical hardware on the computer such as reading and writing to hard drives, accessing the serial ports and putting text onto the screen. Modern operating systems, FreeBSD included, now utilise drivers to access the hardware and the BIOS simply exists to launch them.