This tab displays keywords that are similar to the one you entered. We count two keywords as similar when at least 2 of the top 20 pages are the same for both keywords.
For example, if you search for seo services or seo services company and Surfer finds that at 2 of the top 20 results link to the same page in both SERPs, it will assume that those keywords are similar.
This tab may serve as an inspiration for your future content. However, if the keyword similarity is high, there's a chance you can rank for this similar keyword with your existing page.