This is the 6933rd most frequent Russian word.
анастасии
"Анастасии" is the genitive or dative case of "Анастасия," a Russian female given name meaning "resurrection."
Here, 'Анастасии' is in the dative case, as it signifies possession towards Anastasia.
Книга принадлежит Анастасии.
The book belongs to Anastasia.
In this sentence, 'Анастасии' is also in the dative case, used to indicate who the gift was given to.
Я подарил подарок Анастасии.
I gave a gift to Anastasia.
The word 'Анастасии' is in the prepositional case here, as it is used after the preposition 'об' to indicate discussion about Anastasia.