Organic competitors are the websites which comes in SERP for same keywords as yours, For example ebay.com and amazon.com both comes in SERP for common keywords like online shopping, shopping site, buy online so they are organic competitors.
This tool finds organic competitors of any website (Using Semrush API) and displays them along with Other Semrush metrics. To use this tool simply enter your domain name in box and click Find Competitors button.