After entering your website URL, the site places a grid over your website and shows you how a certain percentage of people around the world are viewing your website, based on data gathered from millions of websites. The opacity of the grid can be changed to allow you to see your website or grid better. It s also important to align your website to the legt or grid using the tool accordingly (depending on how your design is aligned). This will allow you to see the grid correctly.
FoldTester is a simple tool however it is very useful. Millions of websites around the web still have designs which give horizontal scrollbars to a large percentage of readers. This service lets you quickly see what your visitors see. I recommend checking it out.