A seed phrase (also called a recovery phrase) is a human-readable backup of your wallet's private keys, generated once when the wallet is first created. Anyone who has it can recreate your wallet and move everything it controls, on any device, with no further verification needed.
The standard advice is to write a seed phrase down on paper (or stamp it in metal) and store it offline, never typed into a phone, computer, cloud note, or sent in a message. Treat it exactly like a physical key to a safe — because functionally, that's what it is.