typically refers to Open PS2 Loader — a popular homebrew application for PlayStation 2 that allows playing games from USB, HDD, or network shares. However, version 0.9.3 is quite old (released around 2011–2012), and the latest stable versions are much newer (e.g., 1.2.0+).
OPL 0.9.3 significantly improved support for "POPS" (PlayStation On PS2). This feature allows you to play PS1 games via emulation from your storage device. While not 100% compatible with every PS1 title, 0.9.3 made the setup process much smoother.
OPL.ELF file to your PS2 memory card (using Free McBoot) or a USB drive.