A group of 12 to 24 everyday words from which a private cryptographic key is created in a crypto wallet maintained by the user. Also called a "recovery phrase" or "secret recovery phrase" (SRP), the words must be written down and stored in a safe place in case the wallet is ever stolen or damaged beyond repair. In that event, the keys can be regenerated from the seed phrase. Otherwise, all the user's cryptocurrency managed by the wallet is gone forever. See crypto wallet
Seed Phrase from MetaMask
MetaMask is an Ethereum wallet that users must keep secure because the wallet generates the private keys to their ether (ETH).
Seed Phrase from Electrum
The process is pretty much the same for all wallets. The seed phrase is generated and must be backed up.