The GBA BIOS is a set of instructions that are executed before any GBA ROMs are run. It contains GBA system data, GBA hardware registers, GBA memory mappings and GX configuration data.
The GBA was based off the Game Boy Advance hardware, which in turn was based off the Game Boy Color hardware. This means most GBC games can be played on GBAs with no changes to the game code itself; they will only need new system-specific GBA BIOS routines to execute properly.
Some parts of the BIOS reside in separate parts of internal Flash Memory (512Kb at 0xE000 , 2Mb at 0xFDFF ) while others are mapped into main 256K RAM.