。
She swims well.
Literal
She [topic-は] swimming [subject-が] skilled.
うまい is the casual cousin of 上手 — both i-adjective and na-adjective forms coexist for 'skilled / good at,' but うまい (i-adjective) carries a more colloquial, conversational tone than 上手 (na-adjective), which feels more measured. うまい also famously means 'tasty,' so context decides — here the topic is 泳ぎ (swimming), so the skill reading is the only one that fits. The plain form (no copula) gives a casual, quick-remark feel, suited to chatting between friends.