FileVault is a data encryption software that is included in all Apple computers (OS X 10.7 or higher). When FileVault is enabled, all the data on the computer is locked until a password is entered, meaning all data is safe from unauthorized users.
Enabling FileVault is a good protective measure to have in place, should your computer get stolen. Along with requiring a password during login, the only way to access your files will be by entering your login password or whichever FileVault recovery option you choose.
Your iCloud account can unlock FileVault using your Apple ID
You can create a random 24-character code to unlock FileVault
For more information, please visit the FileVault support page from Apple: https://support.apple.com/en-us/HT204837