。
She is very smart.
Literal
She [topic-は] very head [subject-が] good.
頭が良い literally reads 'head is good' — Japanese localizes intelligence to the head (頭) and uses the multipurpose 良い/いい to evaluate it. 頭 takes が as the small-scale subject of the predicate 良い, while 彼女 stays as the broader topic — the classic double-subject pattern that turns an attribute into a predication.