The negative SEO protection is a free badge on your site that discourages people from launching a negative SEO attack on your site. It discourages them because they know it will be ineffective since:
a) they know you're using Monitor Backlinks to get notifications
b) they see that you have many referring domains.
If you want to add it, you can do so by following this link: https://monitorbacklinks.com/negative-seo-protected