Resort Airline Check in.
Lets you check baggage at your resort going home instead of your lugging it to the airport.
Works like the skycap service at the airport.
Not all airlines allow this, also the hours you can do it are limited to 5 AM to 1 PM (most resorts) and also must fit the constraints at least 3 hours and at most 12 hours before your flight.
Don't have the list of participating airlines in front of me but all the major airlines do it except that Southwest doesn't do it from Caribbean Beach or Old Key West.
If your airline has a fee for checked baggage, you have to pay in advance, over the phone. I don't have the number in front of me but this should be available at the resort.
Don't cut it too close. In line to check your baggage via RAC does not count as punctual for the DME bus.