Windows Defender SmartScreen is a security feature built into the Windows operating system that helps protect users from malicious websites and downloads. It works by checking URLs and files against a database of known threats, warning users if they attempt to access potentially harmful content. When a user tries to download a file or visit a website, SmartScreen evaluates the reputation of the source. If the site or file is deemed unsafe, it will display a warning message, allowing users to make informed decisions and avoid potential security risks.