。
She sang beautifully.
Literal
She [topic-は] very skillfully sang.
うまく is the adverbial form of the i-adjective うまい ('skillful, good at'). うまい has an everyday warmth that 上手 lacks — it's the way you'd describe a friend's cooking or a kid's drawing. Its adverbial form うまく often appears in idiomatic uses like うまくいく ('go well, work out') and うまくやる ('do skillfully, manage well'). Pairing うまく with 歌う ('sing') keeps the sentence in casual, admiring register: 'sang really well.' The whole sentence has the texture of someone praising a performance after the fact.