If your referrers have moved to HTTPS and you’re stuck on HTTP, you really ought to consider migrating to HTTPS. Doing so (and updating your backlinks to point to HTTPS URLs) will bring back any referrer data which is being stripped from cross-protocol traffic. SSL certificates can now be obtained for free thanks to automated authorities like LetsEncrypt, but that’s not to say you should neglect to explore the potentially-significant SEO implications of site migrations. Remember, HTTPS and HTTP/2 are the future of the web.
The term “organic traffic” is used for referring to the visitors that land on your website as a result of unpaid (“organic”) search results. Organic traffic is the opposite of paid traffic, which defines the visits generated by paid ads. Visitors who are considered organic find your website after using a search engine like Google or Bing, so they are not “referred” by any other website.