OS X Yosemite: About FileVault encryption
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.
To ensure security when you turn on FileVault, other security features are also turned on. For example, when you turn on FileVault, you need a password to log in when your Mac is in sleep, or after leaving the screen saver. After the initial startup, only users enabled in FileVault can log in; other users need an administrator to log in first. For more information, see Encrypt the contents of your Mac with FileVault.