It means that the content on your website has been organized in such a way that google and other search engines pick it up faster. Certain rules apply to make it the most affective.
for example you want google to pick up your website about cat food. You'd write an article about cat food and mention the word "cat food" in strategic places throughout the article. (Cat food being your main keyword)
A. Main Keyword in the title of the article
B. Main keyword in the first paragraph: 1-2 times, No more than 3 times.
b. Main keyword in the last paragraph: 1 times. No more than 2 times.
This is particularly advantageous to websites that are trying to sell products, or just to get a high google page rank