Recovery phrase
A seed phrase is a sequence of words used by many wallets to derive keys. It can make it possible to restore access to a compatible wallet.
Why it is sensitive
Someone who obtains a seed phrase may be able to reconstruct the associated keys. It is therefore not a normal support, identity-verification or login credential.
Seed phrase vs. password
A password can protect a specific application. A seed phrase, by contrast, may reconstruct the cryptographic structure of a wallet in other compatible software.
Conceptual good practices
Understanding the role of a seed phrase helps explain why it should not be shared with third parties or entered into unrelated forms or websites.