。
She's good at skiing.
Literal
She [topic-は] skiing [subject-が] skilled is.
The XはYが上手だ pattern is the standard frame for 'X is good at Y' — Y takes が as the subject of the skill-predicate 上手 ('skilled'). Self-praising with 上手 is socially awkward in Japan; you typically reserve 上手 for praising others and reach for 苦手 ('weak at'), 下手 ('bad at'), or なんとか ('manage somehow') when describing your own abilities. 上手 is a na-adjective; the kanji breakdown 上 ('upper') + 手 ('hand') hints at an old metaphor of skill as a 'higher hand'.