In brief: Microsoft just introduced a new version of its security software that works across multiple operating systems on computers and mobile devices. The company wants to meet its customers where they are, recognizing that they don't exclusively use Windows.
Microsoft Defender for Individuals is a security suite for Microsoft 365 Personal and Family subscribers, offering protection across all of a family's devices. In addition to the well-rated antivirus and malware protection that Windows 10 and 11 already include, subscribers also get certain defenses on Android, macOS, and iOS.
Android and macOS receive antimalware protection, while Android also gets web protection. There isn't much need for antimalware on iOS due to Apple's strict control over all of the platform's software, so it only gets web protection. However, that's still useful for blocking suspicious links in web browsers and protecting users from sites known to phish or deliver malware.
Subscribers can use a centralized interface to control these features and monitor security across multiple devices. Real-time security updates on a Windows or macOS system can appear on an Android or iPhone app and vice versa.
Users should check the list of regions where Microsoft 365 is available but Microsoft Defender isn't. Defender requires macOS Catalina 10.15 and above, iOS 13.0 or newer, and Android 6.0 or newer. Microsoft delivers the macOS version through a direct download at the official Microsoft Defender page. The others are listed in their respective app stores (linked above).