Whats the difference?

Jitsi Meet and Jitsi Desktop?

Jitsi Meet is a videoconferencing solution that consists of many backend services a web client and there is a mobile client for it.

Jitsi Desktop is the legacy SIP and XMPP desktop client. You can chat and have audio/video calls with other participants that use SIP and XMPP. For it, you need to subscribe to those services, as this is just a desktop client. For years now it is no longer actively developed by the core jisti team and is getting only community contributions.