When enabled, Smart Update allows users to test an update for their website without actually affecting the website itself. Smart Update will create a clone of the website, then complete any pending updates on the clone. From there, it performs some changes to see if there are any issues caused by the updates. The results of the update are made available, allowing you to make an educated decision regarding applying the updates to your website.
If you haven’t already installed or imported an installation into cPanel’s WordPress Toolkit, you will need to do that first to be able to use this feature.