The adverb aboard means on board, as in on a ship, train or plane. Usually the captain will welcome you aboard with a brief speech if you're lucky — or a long one if you're not.
Aboard comes from the French phrase à bord, which has the same meaning as the English word — on board. If you were just hired, your new manager might say "Welcome aboard" (the figurative company train). The phrase "All aboard!" is said as a warning before departure, especially before a ship or train leaves. It means, in essence: haul your keister up here quick or you'll be left behind!
