The Password Vault lets you store all of the user names and passwords associated with your Hosting Account in a secure environment. For instance, you could store the login and password for your WordPress account and MySQL database.
To reset your Password Vault's password, reset your primary FTP user's password (more info).
This article refers to storing passwords for applications in the Password Vault. If you would like information on password protecting web pages and directories see Protecting Your Web Pages With a Password.