jumped into; leapt into에 marks the place or target, and 뛰어든 is the past attributive form of 뛰어들다 (to jump or leap into). The phrase usually modifies a following noun, as in “the person who jumped into the river.”
KO: 강물에 뛰어든 사람을 구조했어요.
EN: We rescued the person who jumped into the river.