System tray is a “legacy” tray where various applications (e.g. Nextcloud, Pidgin and Signal) have an icon with which you can interact with the application without actually opening the main application window. I said “legacy”, because phasing it out was the plan in the Gnome 3 project, but it seems like we’re not getting rid of the tray any time soon, if ever.

In any case the system tray is not enabled by default in recent Fedoras, and the proper way to enable it has changed several times in the recent past as Gnome has been upgraded. This outlines the most recent way to do it, and hopefully this one sticks for longer than a year or so.