The full-disk encryption system called FileVault, introduced way back in Mac OS X 10.7 Lion, keeps the data on your drive encrypted at rest. If your Mac is shut down, a malefactor can’t get at your data—they have to have a password to an account that can start up the system to unlock the encrypted data. That password must be provided when a Mac is booted, and it unlocks a key that in turn unlocks your drive’s data. Otherwise, villains are foiled.
However, FileVault is managed on a per-Mac basis, as it is tied to hardware. If you migrate your data to a new Mac, either through Migration Assistant, restoring from a Time Machine backup, or Disk Utility or third-party cloning software, you can wind up in a state in which macOS thinks FileVault is enabled, but it’s not. The copy or migration is all of unencrypted data, not the underlying encrypted format, because you need the unencrypted data to populate the new computer.
