In short The 12/24-word master key to a wallet. Whoever holds it controls the assets; lose it and nobody can recover it. Store offline, never share.
The words are not a password protecting an account somewhere — they mathematically generate the keys themselves, which is why any wallet app on any device can restore your funds from them, and why anyone who reads them can too. There is no issuer to appeal to and no reset flow. Write them on paper (or metal), keep them off every screen and cloud, and treat every request to "verify" or "sync" them — no matter how official the sender looks — as theft in progress.