A "Combi" is Korg's version of a "Setup" or "Registration" or "Stage Setting" or "User Preset" on every other manufacturer's keyboards.
Basically it's a saved stage setting using layers, splits, arps and/or effects tweaks that you can call up with the simple push of a button once you've set it up.
Korg has a tendency to load a few hundred of these settings into their keyboards while at the same time allowing any of them to be overwritten by the user.
Gary