Google’s Supplemental Index are results for pages with little trust value and thus do not rank compared to regular web pages in Google’s index. Thus, you should make an effort not to have a large number of your site’s pages in the supplemental index. To check for pages in the index, use a query similar to this:
"site:lyrics.rebelpixel.com *** -view"
The example I showed above checks for pages from my lyrics.rebelpixel.com site, just replace the domain with your site’s URL. About two months ago, it numbered to around three thousand pages, but now down to less than a thousand, thanks to the solution provided by Chris Garrett.
But what puts pages in the supplemental index? It can be basically explained by google’s hate for duplicate content. ChrisG explains the “duplicate content issue” further and details the solution you need to implement.
If you haven’t checked your site for this problem, now’s the best time for it. You will definitely benefit from it.