FileVault is a method of using encryption with volumes on Mac computers. Encryption and decryption are performed on the fly.
If you store sensitive information on your Mac, you can use FileVault encryption to protect the files from being seen or copied. For example, if you carry all your company’s financial data on your portable computer, losing it could allow someone to access sensitive data that might hurt your business. If you are logged out when your Mac is lost but the data is encrypted, your information is protected. FileVault encodes the information stored on your Mac so that it can’t be read unless the login password is entered.