Airbnb is an iteration on the company's original name, which was "AirBed and Breakfast." The idea was (and I think still is) that there is a ton of unused potential "capacity" in terms of empty couches and inflatable mattresses (airbeds) that people could offer for rent if given the opportunity.
Air = air travel, BnB = Bed and breakfast
I'm not sure if some people don't know what it means because it might be a British colloquialism or whether it's totally unintentional Airbnb's part.