An esper rating is a representation of a person's psionic potential. Even Humans with the highest esper ratings do not actually have any psionic abilities, but such individuals can acquire them under the right circumstances. Esper ratings are usually determined along with an individual's aperception quotient and Duke-Heidelburg quotient.
Doctor Elizabeth Dehner had a "better than average" esper rating, with a measured value of 089. Lieutenant Gary Mitchell, however, had a higher esper rating of 091, which was deemed "well above average". In fact, in 2265, Mitchell was determined to have the highest esper rating among the crew of the USS Enterprise. (TOS: "Where No Man Has Gone Before")