What are "SUPPLEMENTAL PAGES"?

Supplemental Pages are those pages of your website, Google considers either duplicate (not current) or not unique in content but still Google wants to index that pages or keeps record of that pages, so it marks them as “Supplemental Pages”. Google indexes these supplemental pages but not in regular basis as compare to normal pages. There can be many reasons for why pages go into the supplemental index like duplicate content, distant from root url etc.

You can move your supplemental pages to main index by getting links from pages that are already in the Main Index. Try to get links from pages that don't sell links, not because paid links are bad but because a page that doesn't sell links is more likely to pass value than a page that does sell links. You can easily change the result in 1 week. But Google constantly crawling the web, so if it finds your link removed from that Main index, it again put your page in Supplemental Index.

Thanks & Regards.