Practically speaking, this means your cello must be set in a bulkhead window seat which does not have an emergency exit, and that it will have to be strapped in. Some airlines will provide cargo straps, if you so request. The easiest way to strap in your cello is to place the cello between the armrests, with the back of the case toward the seat. The seat belts should be cinched snugly. There is a wide latitude in interpretation of the regulations from airline to airline and in crew interpretation of those regulations.
Some airlines charge the cello a cargo tax, which is less than the regular passenger tax.
My own experiences while flying with my cello led me to become an expert on this subject. Seven hours and one rented car later, I made it back home. I then called a representative of the offending airline. The vice president for marketing for the offending airline was deluged with letters bemoaning the situation. I anticipated many happy and hassle-free flights.