as boats gets larger and systems get more complex, it's hard for owners to stay on top of things and be familiar with each system. Since larger boats will often have a part time or full time captain on board, (s)he will handle maintenance and repairs, either doing it himself or overseeing repairs.
it all depends on the boat, the captain and what the owner expects from the captain; some captains can (barely) drive the boat and change a racor... others can troubleshoot, maintain or upgrade many systems.
from a marketing standpoint, it means that the boat has a captain (at least part time) and that the maintenance level shoudl be above average. obviously you can bet that some brokers stretch that definition a bit...