I was going through this site http://writing.wisc.edu/Handbook/SubjectVerb.html
and came across the following
Rule :A relative pronoun ("who," "which," or "that") used as a subject of an adjective clause takes either a singular or plural verb in order to agree with its antecedent.
ex: Mary is one of the students who have done honor to the college.
(Adjective clause modifies the plural noun "students." "Students" is the antecedent of "who."
I understand the rule but the example. Can somebody please explain?