Matinee backs itself up automatically, every 3 days, keeping the last 5. The card at the top of Settings says when it last did. Back up now makes one straight away.
Choose a folder (Settings → Backup) to keep backups in Dropbox, OneDrive or iCloud Drive, so they're safe off this computer too. Chrome may ask you to allow it again after a restart; until you press Allow, backups go to Downloads/Matinee.
Restore from file adds what's missing and never overwrites anything newer. If it would bring anything up to date from the backup, it saves a safety backup of your library first. To move to a new computer: back up, install Matinee there, restore.
Replace my library (under a restore's preview) starts afresh from a backup, after making a safety backup first.