Mediocre is an adjective that means "merely adequate" or "of only ordinary quality." A "C" is a mediocre grade for students who are fair to middling.
The roots of the adjective mediocre are from the Latin medial, "middle," and ocris, "mountain." If you think about it, the middle of a mountain is neither up nor down and neither here nor there — just somewhere in between. The definition of mediocre is "of ordinary quality," "merely adequate," and "average." Another word that shares the same root is mediocracy, which means "government run by mediocre people" — an all-too-common condition.