It’s the command line that starts the emulator. Basically, ES is just a nice looking interface that runs things from command line for you.
To break it down the runcommand.sh script does a handful of things, it logs errors and does some cleanup after emulators quit, gives you that little interface to switch emulators and settings, and probably some other things I’m not aware of. The next piece is the actual command line that you would run if you were to start the emulator from command line directly. The part before the equal sign is just the name of the emulator that appears if you hit a button before the emulator were to start.
So what we’re doing is simply adding another command that always runs after you exit the emulator. From the command line you can run multiple commands one after another by stringing them together and separating them by
%ROM% is just a variable that gets swapped out with the path to the rom that’s being started.