。
She's very smart.
Literal
She [topic-は] very head [subject-が] is-good.
頭がいい is one of the canonical 'body-part personality' idioms: literally 'the head is good' = 'is smart, intelligent.' This pattern is hugely productive in Japanese — 気が短い ('impatient,' lit. 'spirit is short'), 口が軽い ('loose-lipped'), 手が早い ('quick / handsy') — body parts as the locus of personality traits. The opposite is 頭が悪い ('dumb'). がいい/が悪い predicates often resist literal translation; learning the whole idiom is more efficient than parsing the parts.