This is the 24th most frequent French word.
leur
"Their" (possessive adjective) or "to them" (indirect object pronoun).
In this sentence, 'leur' is used as an indirect object pronoun, indicating 'to them'.
Je leur ai donné un livre.
I gave them a book.
In this example, 'leur' is again a possessive adjective, showing that the enthusiasm belongs to them.
Je comprends leur enthousiasme.
I understand their enthusiasm.
Here, 'leur' is a possessive adjective, signifying that the house belongs to them.