Generally means that you can still hear the aircraft directly, but you have not heard a recent position for it (perhaps it left mlat coverage).
Only aircraft with a recent position are shown on the map, but the table will show everything you can hear. The exception is that if you select an aircraft with an old position in the table, then it’ll show the last position on the map. When you deselect it, it goes back to the “only show recent positions” rule.
Older dump1090s would show the position age on the detail pane, but we seem to have lost that in skyview (I’ll raise a bug report)