The DTM CSS selector is a tool built into DTM to help users construct selectors. In other words, it will help with targeting elements in Event Based Rules. The goal is to more quickly implement tags on your site.
The DTM CSS selector tool works by loading the page in an iFrame. It may not work in a locally hosted site and also doesn't work when you have to log in to access a page either. For more information on using DTM CSS selector. See this link