This is the 553rd most frequent Japanese word.
それら
"それら" means "those" in English.
In this sentence, 'それら' is used as a demonstrative pronoun to refer to multiple objects ('本' books) in a somewhat distant context.
それらの本はとても面白いです。
Those books are very interesting.
In this context, 'それら' points to a set of clues being discussed or shown, emphasizing they are significant in the mentioned activity.
それらが犯人を捜す手がかりです。
Those are the clues to find the culprit.
Here, 'それら' is utilized to refer to multiple unspecified items previously mentioned or understood in the context.