。
She's good at swimming.
Literal
She [topic-は] swim [nominalizer-の] [subject-が] skilled is.
The nominalizer の turns the verb 泳ぐ ('to swim') into a noun-like unit ('the act of swimming'), then が marks it as the thing she's good at. This is the standard pattern for ascribing ability with verbs: ~のが上手. The formal copula である gives the sentence a measured, written feel; in conversation the contracted ~のが上手だ or ~のが上手です would feel more natural.