What is a wallet passphrase?

A passphrase is an optional password that can added on top of your wallet backup (24-word mnemonic seed). One wallet backup can be used with multiple passphrases, all of which create completely separate wallets. By default, the BitBox02 is not using a passphrase. You can think of that as using an "empty" passphrase.

Before using a passphrase, please read: How does a passphrase work?