。
She's devoted to sports.
Literal
She [topic-は] sports [in-に] is-throwing-self-in.
打ち込む is a compound verb literally meaning 'beat in / drive in' — a hammer driving a nail. In figurative use it means 'throw oneself into / be absorbed in / be devoted to' an activity. The thing one is devoted to takes に. The ~ている form gives the ongoing-state reading: she is in a state of immersion. Common collocations include 仕事に打ち込む ('throw oneself into work'), 勉強に打ち込む ('immersed in study'), and 趣味に打ち込む ('absorbed in a hobby').