。
She's good at imitating her teacher.
Literal
She [topic-は] teacher [possessive-の] imitation [subject-が] is-skillful.
The XはYが construction is at work here — 'as for X (topic), Y (attribute) is Z (predicate).' 'as for her, teacher-imitation is skillful.' This double-subject structure is characteristic of Japanese: the sentence topic (彼女は) and the subject of the adjective (真似が) are different entities. うまい means 'skillful' or 'good at' — more casual than the synonym 上手 (じょうず). 真似 (まね) means 'imitation' or 'mimicry.'