Their automated indexing engines attempt to discern what your page is about so that when a user conducts a search, it will return pages that it feels are most relevant to that topic. If you can make your page appear near the top of the list when users search for topics related to your Web site or business, you'll see all kinds of new traffic.
Headings are the larger print, or subtitles on a page.
Many engines will take the keywords within heading tags and assign them far more weight than words found in the body text. When you think about it, this is only logical since headings often give an overview of the page, or a section topic for the page. The above example assumes Widget is my most important keyword which is why I repeated it in the Title, the Body, and very importantly, in the Heading tag. Use multiple heading tags throughout your page if you like, but always try to stuff them with the keywords you're trying to emphasize. Notice that I purposely used the plural form of the word Widget to double our visibility.