This is the 8702nd most frequent Russian word.
патриарху
"Патриарху" means "to the patriarch" in English.
Here, 'патриарху' is used as a dative case masculine singular noun indicating the recipient of the gift, 'the patriarch'.
Я подарил книгу патриарху.
I gave a book to the patriarch.
Here, 'патриарх' is in the dative case used in the context of involvement with the patriarch's words, expressed as 'to the patriarch's words'.
Монахи прислушивались к словам патриарха.
The monks listened to the words of the patriarch.
In this sentence, 'патриарху' is used in the dative case masculine singular to indicate the recipient of the letter, expressing 'to the patriarch'.