。
She's good at swimming.
Literal
She [topic-は] swimming [subject-が] strong-point is.
得意である means 'is good at' or 'is a strong point' — 得意 (とくい) marks a skill or area of confidence. The formal copula である adds a declarative, written tone. Like 苦手 (weak point), 得意 uses the XはYが pattern: 'as for her, swimming is her strong point.'