Rating is a Monster Play article.
Rating is a fluctuating number that rises and falls based on your victories and defeats in player vs. monster player combat (PvMP). The amount of rating points earned or lost depends on your rating and the rating of your opponent or opponents. A higher rating grants bonus Renown/Infamy from PvMP victories.
From: official Turbine article: Game Systems: Monster Play Overview [03-Apr-2007]