It's one of the add-ons I've been using for the longest time. Its logo works as a led light. It changes color according to the reputation of the site you're visiting, thus warning you of potential threats. Gray means there's no record yet about that site, green means no issue, and so on.
In case of scam or other serious danger, it blocks the page with a banner, which the user may however bypass at his own risk.
Sites' reputation is the result of reports from the surfing community. Clicking on the logo, you can access available details, such as rating, type of threat (scam, adult content, etc.) and users' comments or reviews. By registering you can add your own. As it did, and still does to me, it renders an excellent service to stubborn surfers who tend to be carried away by their mouse.