External backup services are services offering to handle backup storage for data of your site. You can store and backup your site files and databases on any of such services.
It means that apart from the backup you have on your hosting account, you can have another backup residing on a different server and it can help you in retaining your data and files intact.
As you know, it is your responsibility to back up your data. You can have many backups copies of your site or database to make sure you secure your data.
