Focus, But How?

Where does everything else go when one focuses on one thing alone? Is there some place to tuck the myriad voices that cry out for attention? I suppose if one is absorbed enough in one thing, there is no room for anything else.
So is the task of focus becoming completely filled or becoming completely empty? I imagine some of each depending on how your mind works.
In either case, there is not much practical assistance out there except for stating the obvious. "Be more focused!"