A passphrase is like a password only longer: words and characters that you enter to let the app know that the person typing is you.
Crypho also uses your passphrase to form a cryptographic key that is used for fetching other encrypted keys from our servers. It is therefore important that you use a strong enough passphrase. The simplest way of making your passphrase strong is to make it longer.
We recommend using at least three words, for example a short sentence.