RequestPolicy improves the privacy and security of your browsing by giving you control over when cross-site requests are allowed. Cross-site request means that a website makes your browser access a completely different site.
Though usually legitimate requests, they often result in advertising companies and other websites knowing your browsing habits, including specific pages you view throughout the day. Among the attacks that cross-site requests are used in, they are particularly dangerous with Cross-Site Request Forgery (CSRF) attacks where your browser is told to make a request to another website and that other website thinks you (the person) meant to make the request.
With RequestPolicy, the default for any cross-site request is to deny it. Users are notified when requests on the current page have been blocked (the status bar flag icon at the bottom right of your browser turns red). Clicking on this status bar flag icon gives you a menu where you can view and modify which requests are blocked and allowed. You can whitelist requests you approve of by origin site, destination site, or specific origin-to-destination.