The specifically refers to the Asian market version (NTSC-J region, but often with multi-voltage power supplies). This model was a radical departure from the "Fat" PS2s and even the earlier "Slim" (70000 series).
At its heart, the "SCPH-90006 BIOS" is a piece of low-level software hardwired into a chip on the console's motherboard. It is the first code that runs when you turn on the PS2, initializing all the hardware (like the CPU, GPU, and controller ports) and providing a basic operating system environment that game discs can interact with. scph90006+bios+new