Backup is a spare copy of your site. You or your host create a backup of your site.
Then once something happens to your site and troubleshooting it doesn't help, you simply recover your broken site from its backup copy.
The most convenient way to backup a Joomla site for any Joomla beginner is to use the popular and most installed extension "Akeeba Backup for Joomla".
It allows you to create backups of your site right from the convenience of your Joomla control panel. You can easily create them manually, with one click. Or you can set backups to run automatically, on a schedule.
Once you created a backup in your Akeeba Backup for Joomla component, you can use it to easily recover your site.
Simply unpack it to your site root and then run the backup installation process following onscreen instructions. Just as you would do installing Joomla itself.