What does the Do Not Index option in custom pages mean?

What does the “Do Not Index” option in custom pages mean?

The major search engines regularly crawl your site looking for changes which should be noted in their indexes. Occasionally, you may have a page that you don’t want to be recorded in search results. By selecting the “Do Not Index” option, our sophisticated system will automatically apply a unique tag to the page telling the search engines that this page is not to be included.