If you've never been on Product Hunt before, it's like a daily Reddit feed for new products. Products get submitted to the community and they're voted on. Each day products are stacked in descending order based on how many votes they've had. Ranking at the top of the daily list can result in thousands of conversion-focused traffic to your site, just as the creator of Nomad List found out.
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.