boot9.bin is a raw, sector-by-sector dump of the from the Nintendo 3DS’s security co-processor, often referred to as the "Boot9" or "SECURE9" processor. In simpler terms, it is a perfect copy of the very first code that runs when you press the power button on your 3DS.
(often paired with boot11.bin ) is the dumped ARM9 Secure BootROM of the Nintendo 3DS. It represents the "Holy Grail" of 3DS hacking because it contains the absolute first code executed by the system's processor upon powering on, along with the hardware-level encryption keys used to secure the entire platform. Technical Significance The Root of Trust Boot9.bin 3ds