|Definition - When a verb agrees with the nearest noun, rather than with the sentence's more distant subject.|
(1) One of your friends are here.
(This is an example of proximity concord because the verb is agreeing with the noun friends, not with the less-proximate singular subject one.)
(2) One of your friends is here.
(This is an example of grammatical concord because the verb is in proper agreement with the subject one.)