。
She's a great dancer.
Literal
She [topic-は] dance [subject-が] skillful.
うまい is the everyday counterpart of 上手 ('skilled') — both adjectives mean 'good at,' but うまい is i-adjective and slightly more casual, often warmer in feel. The thing one is good at takes が, not を, because 上手・下手・うまい・苦手 behave like stative predicates rather than transitive verbs. So you say ダンスがうまい, never ダンスをうまい. Compare with ダンスが上手 (more polite/neutral) and ダンスが下手 (its opposite), all built on the same が-marked frame.