Microsoft Defender is a built-in security feature in the Windows operating system that helps protect computers from viruses, malware, and other threats. It provides real-time protection, scanning files and applications to ensure they are safe to use. Users can also run manual scans and receive updates to stay protected against the latest threats. In addition to basic antivirus functions, Microsoft Defender includes features like a firewall, which monitors incoming and outgoing network traffic, and Windows Defender SmartScreen, which helps block malicious websites and downloads. Together, these tools aim to create a safer computing environment for users.