To coin a phrase, the curve anchor is the individual, or group of individuals, whose scores make up the bottom of the curve, the weight that pulls the curve down and secures the very bottom of the passing grade.
Poor Johnny got a 25% on the exam. Well, Johnny, don't feel too bad. Lift your head up! While you may not have done so well, my C sure looks better than I ever thought it could! Thanks for being the Curve Anchor!