This is the 9312th most frequent Japanese word.
目星
A rough guess or target; an aim or clue.
Here, 目星 is used to indicate a target or a choice someone has decided upon.
彼女は自分の目星をつけた店でショッピングを楽しんでいる。
She enjoys shopping at the store she targeted.
In this sentence, 目星 refers to identifying a clue or something significant as a focus for action.
警察は事件の重要な手がかりを目星として調査を進めている。
Police are advancing their investigation using the key clue they identified.
Here, 目星をつける means to pinpoint or determine something as a target or an objective.