This is a plugin for PSPs running Custom Firmware.
When you load a PS1 game in PPSSPP or RetroArch with this BIOS: psxonpsp660bin bios
When Sony developed the PSP, they created a built-in PS1 emulator (often referred to as "POPS") to allow users to download and play PS1 Classics from the PlayStation Store. To make this work, Sony included a specific PS1 BIOS within the PSP's own system software. The file commonly known as psxonpsp660.bin is that specific component extracted for use by the homebrew community. This is a plugin for PSPs running Custom Firmware
For specific PS1 games, this BIOS actually works better than standard PC BIOS files. The file commonly known as psxonpsp660
Sony included a built-in, high-compatibility software emulator for PS1 games inside the PSP’s firmware. That emulator required a copy of the PS1 BIOS. The psxonpsp660.bin file is that exact BIOS dump.
It is important to note that BIOS files are copyrighted software belonging to Sony Interactive Entertainment.