We send each user an email with a backup phrase when they sign up for an account. It should be twelve real words, that are not connected as a sentence.
You'll need that in order to unlock your account in case you ever forget your password or get locked out.
We use a backup because it's a way to better protect the documents you store using Vaultie. Some people might know your mother's maiden name or first pet which doesn't keep your personal documents safe. Also, this phrase serves as a backup for your cryptographic key used to sign documents.
We advise you to store that phrase somewhere safe, perhaps even written somewhere in case you ever need it.