You might have noticed that a lot of apps on your Galaxy Tab 10.1 were designed for much smaller devices. So the Tab stretches to fill screen and that sometimes makes things a little iffy. Google is trying to remedy this by giving an option between Stretch to fill screen to Zoom to fill screen.
So what is the difference between the two of these? Stretch keeps all the elements the same size without pixelating them. Zoom is very similar to what the iPad uses to display phone apps on the tablet. It literally will zoom in and cause much pixelation. This can be good for some apps and unnecessary for others. That is why you have a choice.
This will be available in a new update (3.2?) soon. What do you think about this new feature?