An esper rating was a representation of a person's psionic potential. Even Humans with the highest esper ratings did not actually have any psionic abilities, but such individuals could acquire them under the right circumstances. Esper ratings were 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")