With FileVault on, a Mac can’t be used without a password. It's a good security step to take if you have sensitive files that would be risky for others to view.
To turn on Filevault, click Finder > Security & Privacy > FileVault.
Click Turn on FileVault. Select the user accounts that are allowed to unlock the encrypted drive and enter a password for each account.
FileVault recovery key
You’ll be given a recovery key to unlock your Mac should you forget your FileVault password. Write this key down and store it safely.
You can also store your recovery key with Apple via your iCloud account. To do this, you must provide three security Q&As.