。
She's good at tennis.
Literal
She [topic-は] tennis [subject-が] skilled.
The minimal three-particle skill claim: は marks the person, が marks the skill domain, うまい states the level. Drop the intensifier and you have the rock-bottom version of '[person] is good at [activity].' うまい (i-adjective) here is preferred over the slightly more formal na-adjective 上手 in casual speech; both grammatically pattern the same way. うまい also covers 'tasty' for food, but 上手 stays exclusive to skill — a small but useful asymmetry between the two near-synonyms.