The similarity score, introduced by the Society for American Baseball Research. compares the similarity between a current football players statistics and those of a Hall of Fame player he is similar.
Similarity scores are among the many original sabermetric concepts first introduced by Bill James. In sabermetrics and basketball analytics, similarity scores are a method of comparing baseball and basketball players (usually in MLB or the NBA) to other players, with the intent of discovering who the most similar historical players are to a certain player. James initially created the concept as a way to effectively compare non-Hall of Fame players to players in the Hall, to see who was either on track to make the HOF, or to determine if any eligible players had been snubbed by the selection committee. 
