。
She's a good swimmer.
Literal
She [topic-は] swimming [subject-が] skilled is.
Here 泳ぎ is the masu-stem of 泳ぐ used as a noun ('swimming as a noun-like activity'). This kind of stem-nominalization is productive for many activity verbs — 釣り ('fishing'), 笑い ('laughter'), 始まり ('beginning') — though not all verb stems make natural nouns. The double-subject construction XはYが上手です ascribes the skill of swimming to her as her attribute, the standard pattern for skill descriptions.