The supplemental index is Google's database of pages deemed less important than the pages residing in the primary index.
A supplemental page can still rank, but only if there aren't enough results deemed as quality enough for the primary index. Google used to flag pages from the supplemental index when they were served to users. However, this practice died out in 2007.
Google has revealed a separate mobile index that will be given preference which also reveals the existance of multiple indexes. Google has been quiet over the existence of multiple indexes since the supplemental index went dark in 2007. However, Google's John Mueller said in 2014:
"We don't have a supplemental index any more in the sense that these pages will be treated differently in the search results. So that's not something you'd need to worry about.
"We do have different index tiers, depending on how we categorise your pages, how we need to crawl them, but it's not something you'd see specific changes in the search results."