Error when booting Mac with FileVault Encryption
What is really is might be a bit of a guessing game, but the next step is clear.

Boot to your Recovery HD and see what disk utility makes of the volume itself.

If you lack a Recovery HD - then it's time to make a back up of any files you can reach (attach the drive to a sled / boot in target mode / install the OS to an external drive and copy things off using another OS.)

At that point, you can re-run the installer which may fix the issue for you and will certainly get you back to having an Recovery HD or expose a hardware problem if you can't even re-instal the OS on top of the data that is there.