Google’s search results are now set up in a two-tier system. You have the main, or regular search results, and the supplemental results. Supplemental results will generally show up at the very end of keyword search results. The odds that your ’supplemental’ page will be available to someone performing a search isn’t very good.

Let’s see Google official definition from their webmaster page

"A supplemental result is just like a regular web result,
except that it’s pulled from our supplemental index. We’re able to
place fewer restraints on sites that we crawl for this supplemental
index than we do on sites that are crawled for our main index. For
example, the number of parameters in a URL might exclude a site from
being crawled for inclusion in our main index; however, it could still
be crawled and added to our supplemental index."

To find out how your own website (or a competitors site) is doing in the two-tier system go over to Google and type in site:www.yourdomain, of course you will have to put in your actual url. If your pages are relegated to the supplemental results you will actually see “Supplemental Result” written to the right of the url below the description in the listing.

The most common reason that your pages may be in the supplemental results is duplicate content. Pages with little, or no content will also be placed in the supplemental results. Another possibility is that you may have identical title and Description meta tags on all or many pages.

The best advice is to take actions to avoid having your pages placed in the supplemental results in the first place. However, it is possible to get pages back into the main listings.

Make a serious effort to provide unique and original content on all your pages. If in question you may check your pages for originality at copyscape.com. Write a unique title and description meta tag for each page. If you have pages that consist mostly of images add text so that Google will ’see’ content. There are different opinions as to how much content you need on a page but 500 words or more is best.

When you work on your inbound links do not make all of the links point to your home page. Whenever you have the opportunity get backlinks to internal pages of your website. The lack of inbound links or having only reciprocal links has often been pointed to as a cause of pages being placed in supplemental results.

Finally, don’t give up or lose hope. Most all websites have pages listed in the supplemental results. Concentrate on providing good original content as that is, by far, the most significant factor affecting how your page will be treated by Google’s two-tier system.