Inboard means inside a boat — toward the middle of the hull, not hanging off the side. An inboard engine sits inside the boat; the contrast everyone uses is an outboard motor, which hangs on the back. As an adjective it names where something lives (an inboard engine, the inboard side). As an adverb it names the direction: you move a box inboard when you shift it away from the edge and toward the center. Sailors and people who work on boats use it; most learners meet it next to outboard, talking about engines. Though actually, the same picture is borrowed for planes and even cars: inboard can mean toward the middle of the body, not the wing-tip or the outer wheel. That is still the “inside, toward the center” idea, just not only for boats. It is a practical word, not a fancy one.