means A small shaped object, usually metal, that operates a lock — or, by extension, anything that unlocks, controls, or explains something (a key fact, a key to the code, the key of a song).

from From Old English 'cǣg,' meaning a key or solver of riddles — a word whose deeper ancestry is genuinely murky, with no confident relatives in the other Germanic languages. So the metal object came first, and English happily lent its name to every figurative 'key' that opens, unlocks, or decodes — the musical key, the map key, the key to your heart — all of them borrowing the quiet promise that some small thing can let you in.