What is a passphrase?

A BIP39 passphrase is an optional extra secret added to your 12 or 24 words. It is sometimes called the 25th word, but it is not on the word list and it can be any text. The words plus the passphrase make the wallet. The same words with no passphrase, or a different one, make a different wallet.

There is no wrong passphrase

Every passphrase opens a valid wallet. The words have a checksum. The passphrase has none. Type it with one letter different and the wallet will not tell you it is wrong. It will open a different, empty wallet. Capitals, spaces and punctuation all count. Test it the way you test the words: with a restore, on the same kind of device, before the wallet holds more than a small amount.

What it protects against

If someone finds or photographs your words, a strong passphrase that is kept somewhere else leaves them with an empty wallet. That is its job: protecting you when the backup of the words is found.

It does not help if you type the words and the passphrase into a fake app or website, or approve a malicious transaction yourself. Many thefts work that way. Habits matter more there: never type the words into a phone or computer, and read what you sign on a hardware wallet’s own screen.

The cost

A passphrase is a second secret to back up. Lose it and the coins are gone as surely as if you had lost the words. Write it on paper or metal, never in any digital form, and store it apart from the words so one find does not reveal both. A short or guessable passphrase adds little: someone who has the words can try common ones quickly.

Whoever inherits your coins will need both secrets, and needs to know a passphrase exists at all. Plan for that before you set one.