JSON is a full backup — it can be restored. CSV is for opening in Excel and cannot be restored. Your photos live in separate storage; a backup holds only the links to them.
Stored backups
The last 20 backups are kept and older ones are deleted automatically. An automatic backup is taken every Monday and emailed to you.
Restore from a file
Loads a JSON backup you downloaded earlier. Current data is deleted and replaced by the backup — but another backup is taken first, so you can go back.