All Vault data is encrypted by Bitwarden before being stored anywhere. To learn how, see Encryption.
Vault data can only be decrypted using a key derived from your master password. Bitwarden is a zero knowledge encryption solution, meaning you are the only party with access to your key and the ability to decrypt your Vault data.
Vault data that is encrypted includes, but is not limited to:
Names of Folders, Collections, Items, & Attachments
All Login information (including usernames, passwords, URIs, TOTPs, etc.)
All Card information (including cardholder name, number, brand, expiration, security codes, etc.)
All Identity information (including names, email, phone, passport numbers, license numbers, SSNs, addresses, etc.)
All Secure Notes and notes connected to a Login, Card, or Identity
All Custom Field name/value combinations