。
She sings well.
Literal
She [topic-は] song [subject-が] skillful-is.
Familiar pattern, simple sentence: X が うまい = 'good at X.' The thing one is good at is marked with が because うまい (and 上手, 下手, 得意, 苦手) are state predicates — they describe a relation between speaker and activity, with the activity functioning as a kind of subject. うまい has a slightly warmer, more colloquial flavor than 上手 (for someone's skill, it's a touch more affectionate; the same word also covers 'tasty' for food, from the same root of 'good, fine'). A clean, breezy compliment.