The Free/TrueNAS OS lives on a live ZFS pool, and that's been the case since the release of 9.3, about 6 years ago. If your boot device fails, reinstall the OS and upload a copy of the configuration database you've hopefully saved.
No it's not, Be careful for if you used encryption or even a passphrase you can loose everything without a backup. Always back up your configuration and your keys, even if you don't think you will need it, you will need it eventually. Get in the habit.
As a newbie read/glance the manual and poke around all the Truenas menus and settings before you commit data.
Very good point; I hadn't considered encryption. @jacksonG, if your pool is encrypted, you'd need to have a copy of your encryption keys; without them your data will be permanently lost.