It's essentially a "heat map" that will show where you might have some distortions or stretching in your UV islands. Stretching, of course, can effect how textures are applied to your mesh, and having Blender analyze your UV islands rather than just trying to eyeball things, or fiddle with things, is much, much easier. As always, Blender Guru explains it best. You can just skip to the 11:00 mark.
If you are having problems seeing the CAPTCHA, please clear your browser's cache and cookies and ensure that you do not have any browser addons or extensions that interfere with the display of the CAPTCHA. Then, close the CAPTCHA window and click on the register button to open the CAPTCHA again.