This is the 7466th most frequent Russian word.
даниилу
"Даниилу" is the dative case of the male name "Даниил," meaning "to Daniil" or "for Daniil."
In this sentence, 'Даниилу' is the dative case form of the proper noun 'Даниил'. It is used here to indicate the indirect object of the verb 'дал' (gave), specifying to whom the book was given.
Я дал книгу Даниилу.
I gave the book to Daniil.
Here, 'Даниила' is in the accusative case of 'Даниил', marking it as the direct object of the action 'смотрели' (were looking).
Мы смотрели на Даниила через окно.
We were looking at Daniil through the window.
In this instance, 'Даниил' is in the nominative case, acting as the subject of the sentence, performing the action of identification with 'есть' (is, implied in Russian).