This is a secondary index used by Google. When you use the “site:” tool when doing a Google search on your site. (site: www.topwebsitetips.com) pages that Google has in their supplementary index are listed as “Supplemental Result”.
Supplemental results generally do not show with the main searches so it is a bad thing to have any important pages of your site listed there. In most cases this is reserved for old, orphaned or broken pages, although there are a growing number of relevant pages that are showing up there.
Much has been written about this, and it appears to be a growing problem with many websites.