A back-up version of your website is a copy of your current, working website. Before you begin making major changes to your website, it is always a good practice to periodically back up your website.
Backups will come in handy if you find you are making changes to your website on a regular basis and decide you want to return to a previous version of the site. Or perhaps you would just like to look at how your site is performing now versus how it was performing before you made the any changes.