Apps that live in ... um, that thing in whichever corner of your screen, with all the icons in it.
For Windows users, the system tray is at the bottom right of the taskbar when the taskbar is attached to the bottom of the screen. On Linux, the equivalent notification area varies according to the window manager, but most default it to either top right or bottom right. On macOS, it lives at the top right. (It's almost invariably near the clock on all OSs.)
Anyway, the point of the system tray, or whatever your OS calls it, is to store the icons of things that are quietly running in the background. Some Stack Exchange apps live here - mostly desktop notification managers and the like. This tag keeps them all together.