BIOS files enable your ROMs to run more accurately. In most cases, you do not need to provide them in order to play a game, but some emulators do require them. For legal reasons, we can't tell you how to get your hands on BIOS files. However, if you have them, you can create a folder called "bios" on your USB and chuck them in there. Then you can point the emulator within RetroArch that requires a BIOS file to the necessary file on your USB.