The BIOS acts as the firmware that translates code between the game and the hardware (or emulated hardware). While many modern PS1 emulators (like DuckStation or Beetle PSX HW) can use "HLE" (High-Level Emulation) to run games without a BIOS file, using a real ROM like ensures:
Emulators are often case-sensitive and require specific filenames to boot games. The most common BIOS file used is the North American version. Rename to: SCPH1001.BIN ps1-rom.bin bios