This follows the advice of "Keep it Brief." If you have had to give a Power Point presentation you may have learned the 6 x 6 rule. Basically humans can digest a certain amount of information at a time. While we can usually remember 7 digit phone numbers, we often have trouble with any datum beyond 7. Therefore on a Power Point slide the rule was to not make more than 6 point on a single slide, and each point should only contain 6 words. If you exceed this, people will not retain the information.
What does this have to do with web design? Look through your site. Each page should address one major topic such as the history of your business or the services you provide. Within each page there should only be a maximum of six headings or sub-topics. While your paragraphs will certainly contain more than 6 words, your headings should not exceed six.