This is the 9164th most frequent Russian word.
сергею
"Сергею" is the dative form of the male name "Sergey," meaning "to Sergey" or "for Sergey" in English.
In this example, 'Сергею' is in the dative case, marking the indirect object, the one who receives the gift.
Друзья подарили подарок Сергею на день рождения.
The friends gave a gift to Sergey for his birthday.
In this sentence, 'Сергею' is in the dative case, indicating to whom the book is handed.
Я передал книгу Сергею.
I handed the book to Sergey.
Here, 'Сергею' is in the dative case again, showing the recipient of the action (receiving the letter).