This is a quick easy way to get a feel if you website (and associated graphics) are visible on most everyone’s browsers. There are many different screen resolutions and if you make your sight too big, it may look okay to you, but what about the other 80% of the people out there?
This utility from Google Labs (http://browsersize.googlelabs.com/) allows you to apply & see what your site would look like by generating an overlay that graphs the statistics of various browsers captured by Google. For example, the “90%” contour means that 90% of people visiting Google have their browser window open to at least this size or larger.
If you have a “donate now” button which falls within the 80% contour, meaning that 20% of users cannot see this button when they first visit the page. 20% is a significant number; knowing this fact would encourage the designer to move the button much higher in the page so it can be seen without scrolling.