In other words it is difficult for the human eye to clearly see content if the relative difference between light and dark areas is not large enough. So if the colors on the website are too simular the site will be difficult to read. Make sure there is enough of a difference in colors that your website content is very easy to view.
Size contrast differences effect where and how we look at objects. Large objects demand more attention and smaller objects may be overlooked. A large difference in size can draw more attention to the larger objects.
It is difficult to read when similar colors are in close proximity. It is much easier to read complimentary colors when they are near each other.
Common Methods of Creating Contrast