Hackers use cross-site scripting, also known as XSS, to gain control of the content of your web pages. They do this by entering a piece of malicious code into your site, usually via an existing input field, such as a search box, or name/address box. If your website is not configured to combat this type of invasion, the hacker will be able to take control of your site, including collecting personal information from your unsuspecting customers.
SiteLock's 360 Degree Deep Scan will test each of the input fields on your site by inserting code into them, just like hackers would do (but SiteLock code is harmless!)
To avoid an XSS attack, make sure all your apps and code are up to date, where possible reduce your risk from third-party plugins. If the SiteLock scan finds a vulnerability, you will need to quickly verify and harden your code for security.