A seed phrase (also called recovery phrase or mnemonic phrase) is a series of 12-24 words that serves as a master backup for a cryptocurrency wallet. It can restore access to all accounts and funds.
Seed phrase standards: BIP-39 defines word lists, uses 12, 15, 18, 21, or 24 words, derives all private keys deterministically, and supports multiple cryptocurrencies from one phrase.
Security best practices: Write down on paper or metal (never digitally), store in multiple secure locations, never share with anyone, and test recovery before storing large amounts.
Anyone with your seed phrase has complete access to your wallet. There is no way to recover funds if the phrase is lost or stolen.