PerfRank is a shortened version of Performance Ranking.
PerfRank is an indicator that allows you to rate how good or bad your ranking is, i.e. your performance. The PerfRank ranks your performance from 1 to 100, 1 being the best and 100 being the worst.
The score is assigned to you according to the search volume that the ranking receives.
The Perfrank will reveal how well your keywords are performing according to their search volumes. The better your site ranks on keywords with a high search volume, the closer your score will be to 1.