The score is based on a simple measure: The number of other rings each site in this ring belongs to. If, on average, each site also belongs to many other rings then this ring is not considered very unique. If you have visited several other rings on the same topic you may well have already visited many of the sites in this ring.
A high score indicates that the sites in this ring are not likely to be found in other rings, even within the same topic area. A score above 75 indicates a ring with high uniqueness (you aren't likely to find these sites elsewhere in WebRing), 50 indicates good uniqueness (you can find some of these sites elsewhere), and a value under 30 not exceptionally unique (you're likely to find many sites in other rings).