Want to quickly zoom in on a folder to see its contents better? Switch to the select tool, the one that looks like an arrow, and then aim the mouse at the folder you want to see in greater detail. Double-click on its titlebar and it'll expand to fill the display (or most of it)!
And if you double-click on its titlebar again, you'll zoom back out. Neat, huh?
The reason this is done with the select tool instead of the zoom tool is that you most often will be using the select tool anyway, so this provides a quick way to zoom in without switching tools.