Related % is based on the URLs being displayed in the top 100 results for the keywords you are analyzing. This metric compares the main searched keyword in SEMrush's first ten pages of SERP results (what you searched in the top search bar) to the related keyword's first ten pages of SERP results. Depending on how these URLs relate, you will receive your Related % metric.
For example, lets say you search the seed word "movie" in the Related Keywords report and you compare it to the related keyword "what movies are out in theaters" shown in the results. If the Related % is 40, this means that 40% of the URLs that are displayed in the top 100 organic results for "movie" are also displayed in the top 100 organic results for "what movies are out in theaters."