When you enter Mission Control, an application's windows are stacked atop each other. If you're after a particular window, but it's not the frontmost one in an application, it could be difficult to identify it.
To solve this, after you've entered Mission Control, move the mouse cursor over a stack of windows and scroll up. They fan out somewhat from each other, allowing you to see more of the windows' contents. Other applications and the Desktop dim.
To exit this mode, either scroll down or click off in one of the dimmed areas.
Oddly, Apple did not choose to use the 'pinch out' gesture for this feature. This is unfortunate, since it's a much more intuitive way of commanding stacked items to spread out.
[crarko adds: I tested this, and it works as described. I tried the pinch out gesture with the Magic Trackpad (in 10.7.2), and it still didn't work, like the hint says.]
Mac OS X Hints