means An unobstructed space, gap, or hole through which something can pass — or, by extension, a chance or available position that lets you move forward.

from From the verb 'open,' which traces back to Old English 'openian,' meaning to make open or unclose. That verb is tied to the adjective 'open' (Old English 'open'), a cousin of words across the Germanic family — Dutch 'open,' German 'offen' — and probably related to the idea of 'up,' as in something lifted or raised away to leave a gap. The '-ing' ending turned the action into a thing: first the literal hole, then, by easy metaphor, the figurative one — the vacancy, the chance, the moment you either seize or don't.