"In focus" means that the particular window is activated and ready for input rather than just showing on the screen.

Typically but not always indicated by the bar at top being Blue for In Focus. Grayed bar is out of focus. This may change colors if you've messed with the Windows presentation scheme, but the same logic will apply.

So you click on that bar to bring the window into focus if there is any other window that has the bar turned dark blue instead.


--Mac