What is the meaning of Grade_on_a_curve?

To use a grading system based on the scale of performance of a group to normalize a right-skewed grade distribution (with more lower scores) into a bell curve, so that more can receive higher grades, regardless of their actual knowledge of the subject; to curve a grade.

To compare the members of a set or group by using a relative scale, based on performance relative to other members.

Source: wiktionary.org