差 (chà) — (adjective) poor, inferior, not up to standard; (adjective) wrong, mistaken; (verb) to be short of, to lack

With tone chà it covers two main uses: as an adjective meaning 'wrong' (你写差了, like 错误) or 'poor/inferior', and as a verb meaning 'to be short of' (还差两块钱, like 缺). Don't confuse it with the other tone chā meaning 'different'.