Part of macOS
SystemUIServerMenu bar items
○ Leave it
Runs some of the menu bar’s status items.
What it is
Hosts menu bar extras that belong to macOS.
If you stop it
Those menu bar items vanish for a moment and macOS restarts it.
The verdict
○ Leave it. Part of macOS, or something that starts again by itself. You never need to close these.
In Activity Monitor
SystemUIServer
From Procession’s guide, in the words the app shows.