。
She put the key in her pocket.
Literal
She [topic-は] key [object-を] pocket [in-に] put.
A textbook A-を-B-に-入れる pattern: the moved object takes を, the destination takes に. ポケット is from English 'pocket' — written in katakana like all foreign-origin words, but so naturalized that no native speaker thinks of it as borrowed. Many clothing-related loanwords (シャツ, スカート, ズボン, ボタン) entered Japanese during the late-nineteenth-century push to adopt Western dress.