Microsoft Defender is the built-in anti-malware package that’s included with modern Windows operating systems. It’s alternatively known as Windows Security (it shows up under Settings > Privacy & security as Windows Security) or Windows Defender (sometimes with Antivirus at the end of the name, as in this Microsoft Learn page). But whatever you want to call it, for many Windows users, this tool is the go-to default for handling security on their PCs.
As with Windows Update in general, sometimes Microsoft Defender updates may not work. Normally, Defender updates are handled as part of routine Windows update behavior, run on a daily basis as a scheduled task. But sometimes, Windows Update itself runs into problems and doesn’t do much (or anything).