A Seed Phrase is human-readable recovery material from which compatible Bitcoin wallets derive keys. Anyone who obtains it can usually control the funds; losing every usable copy can make recovery impossible. Keep it offline and never send it in a message, to a website or through a support form.
In BIP39, ordered words encode randomness and a checksum; together with any passphrase, the wallet derives the recovery basis for keys and addresses. This is not a password a company can reset. Do not invent the words yourself or translate them into another language. [BIP 39: Mnemonic code for generating deterministic keys]
BIP39 converts, for example, 128 bits of randomness and 4 checksum bits into 12 words; 256 and 8 bits become 24 words. The checksum detects only some errors: it does not establish safe generation or wallet ownership. The words and any passphrase then derive a 512-bit seed, which is distinct from the original randomness. [BIP 39: Mnemonic code for generating deterministic keys]
An optional BIP39 passphrase changes the derived wallet. A different or mistyped passphrase also produces a valid but different result, often without the expected funds. Back it up separately and accurately; a provider cannot reset it to restore the original wallet. [BIP 39: Mnemonic code for generating deterministic keys]
Not all word backups use BIP39. Preserve information about the format, address type and accounts or derivation paths used. With Multisig, cosigners’ public keys and the spending condition also matter, often described by an Output Descriptor. Correct words alone therefore do not prove another application will find all your funds. [BIP 380 — Output Script Descriptors]
A Seed Phrase allows wallet keys to be derived again. Treat its disclosure as potentially handing over control, not merely losing privacy. If a passphrase is used, it is also needed for the same wallet; with Multisig, spending still requires the specified number of signatures. [BIP 39: Mnemonic code for generating deterministic keys] [BIP 380 — Output Script Descriptors]
Store complete, readable copies with fire, water, discovery and coercion risks in mind. Do not photograph, email or put the words in ordinary cloud storage. Durable material does not replace control over access to the backup. [Bitcoin.org — Securing your wallet]
Use the manufacturer’s verified procedure on a trusted device. If a Hardware Wallet offers an on-device backup check, use it; do not erase your only working wallet just to test. During recovery, verify expected addresses or a small controlled balance. Never enter the words at the request of an unsolicited person or website. [Trezor — Check wallet backup on Model T]
A complete backup can resolve the loss of a device. Exposed words are a different problem: restoring the same words or changing the PIN does not revoke someone else's copy of the keys. Endangered funds require a safe move to newly generated keys; with Multisig, the procedure depends on the entire signing policy. Restoring keys does not recover funds already stolen. [BIP 39: Mnemonic code for generating deterministic keys] [BIP 380 — Output Script Descriptors] [Trezor — Move to a new wallet backup]
For the clearest picture, read this entry together with BIP 39, Bitcoin wallet, BIP39 passphrase. The reverse links also lead from BIP 39, HD Wallet, Cold Storage, Hardware Wallet.