Sorry to laugh, I'm not laughing at you but at the state of tech writing in general today.
The "focus" here means the active window, it's a Windows term (or to be more exact, a windowing term), the window where something happens when you enter something on the keyboard perhaps, and more generally where the foreground application process runs, which has implications for how much CPU power it gets (based on internal settings that you can adjust).
basically if u mov out of vegas, yur imeline will retain the thumbs of ur video track frames, but strewn across it will be "media offline"
jump back into the vegas and the clips ceome active again
This also happens when capturing HDV and you sit idle...
i think its a good ssytem as it shows a good use of system resources as u work on other things while keeping the timeline open and active, all teh while the OTHE Rapp ur using is running at its optimal and not bogged down with Vegas...
I cnt say other NLE ive worked is as efficient with resources as this...