This is the 825th most frequent Hebrew word.
להן
To them (feminine).
Here, 'להן' is used to mean 'for them (feminine)', referring to the books being suitable for specific persons or groups.
הספרים להן מתאימים לנושא השיעור.
The books for them fit the lesson topic.
In this sentence, 'להן' means 'to them (feminine)' and indicates that the offer was directed towards a feminine plural group.
הציעו להן להשתתף בתחרות האמנות.
They were offered to participate in the art competition.
Here, 'להן' functions as 'to them (feminine)', referring to the recipients of the explanation.