What is mixed content?
When you visit a page served over HTTP, your connection is open for eavesdropping and man-in-the-middle attacks. Most websites are served over HTTP because they don't involve passing sensitive information back and forth and do not need to be secured. When you visit a page fully served over HTTPS (gray padlock or green padlock in the address bar), like your bank, your connection is authenticated and encrypted and hence safeguarded from eavesdroppers and man-in-the-middle attacks.
However, if the HTTPS page you visit includes HTTP content, the HTTP portion can be read or modified by attackers, even though the main page is served over HTTPS. When an HTTPS page has HTTP content, we call that content “mixed”. The page you are visiting is only partially encrypted and even though it appears to be secure, it isn't.
The Mixed Content Blocker blocks potentially harmful HTTP content on HTTPS pages.
What are the risks?
An attacker can replace the HTTP content on the page you're visiting so that they can steal your credentials, take over your account, acquire sensitive data about you, or attempt to install malware on your computer.