This is the 2728th most frequent Russian word.
даниил
"Даниил" is the Russian form of the name Daniel, meaning "God is my judge."
In this sentence, 'Даниил' is a proper noun used as the subject of the sentence. It is the name of the person performing the action.
Даниил читает интересную книгу.
Daniil is reading an interesting book.
Here, 'Даниил' again is a proper noun, referencing the same individual, but now it appears as part of a descriptive clause defining the book in past context.
Мы поговорили о книге, которую читал Даниил.
We talked about the book that Daniil was reading.
In this usage, 'Даниилу' is the dative form of the proper noun 'Даниил', indicating the indirect object of the action (the recipient of the gift).