means Small shaped pieces of metal (or their digital equivalents) used to open locks, start engines, or grant access — and the set of them you carry around and inevitably misplace.

from From Old English 'cǣg,' a word for a key or solution, whose deeper roots are genuinely murky — it doesn't have clear cousins in the other Germanic languages, so where the Anglo-Saxons got it is something of a linguistic locked door. The figurative sense ('the key to success,' a 'key' role) is ancient and intuitive: whatever opens the way is the key. The musical 'key' and a keyboard's 'keys' borrow the same image — a lever or note that unlocks sound.