The percent of cases that are at or below a score.
But only count half of the cases that are at the score.
Example: in a recent test 8% got A, 30% got B, 50% got C and 12% got D
If you got a B then your percentile rank is half of the 30% that got B plus the 50% that got C and the 12% that got D, for a total percentile rank of 15%+50%+12% = 77%
In other words you did "as well or better than 77% of the class"
(Why take only half of B? Because you shouldn't imagine you got the "Best B", just an average B)