DEFINITION
12 or 24 ordinary words that back up every key in your wallet. Anyone with the words has the bitcoin; anyone without them, including you, cannot recover it.
The phrase (BIP-39 standard) encodes the master secret from which all your keys derive. Any compatible wallet, anywhere, forever, can rebuild your wallet from it — device death, house fire, or bankruptcy of the manufacturer notwithstanding.
Handle accordingly: write it on paper or stamp it in steel, store it where fire and family risk are addressed, and never type it into anything connected to the internet. Every "wallet drained" story traces to a photographed, typed, or phished seed.