It’s a favorite term at one professor’s lectures, ” the key point to take away…” or in a sales presentation or an article- “key to the success…” Always followed by paragraphs of complexities.
But a key is a simple thing like the keystone in an arch. Remove the keystone, and the arch collapses. Simple.
A key locks or unlocks. Not like the reams of paper that only confuse. That seems key.