Content Security Policy (CSP) allows you to dictate a policy for content restrictions on a web site that is enforced by the browser. By setting a CSP header, can control the resources that are loaded when a visitor is viewing your website. The CSP spec allows you to build very specific policies for your site that can handle all of the different types of resources that a site might load. It is a valuable tool for protecting your site visitors.

In addition to blocking resources that violate the defined CSP, the CSP spec details a mechanism for sending reports of such violations. Monitoring these violations can help you ensure that your site is never serving content that violates your wishes whether those violations are the result of producer, developer, or hacker actions.

Source: What to Expect When Expecting Content Security Policy Reports