What is backed up, and when
An automatic backup runs every night. It captures your home directory — including public_html — your cron jobs, and a dump of each of your MySQL databases.
Backups are kept for 7 days, plus one per week for 4 weeks.
You can take an extra backup at any time from the Backups tab. Do that before a risky change: a plugin update, a theme change, a bulk delete.
Restoring
Choose a backup and select Restore…, then choose what to put back: Everything, One database or One file or folder. Restoring everything overwrites your current files, databases and cron jobs with the contents of that backup.
Read that sentence again before you use it. Anything created since that backup was taken is lost. If you are unsure, take a fresh backup first — then you can get back to where you are now.
Downloading
Each backup can be downloaded as a tar archive containing your files and the database dumps. Keeping a copy somewhere else is a good habit, and it is the only copy that survives if your account is closed.
When a container is deleted
Destroying a container purges its backups too. Download anything you want to keep before cancelling the service, not after.
Step by step, with screenshots: Container Hosting Backups: Back Up, Download and Restore.