This is the 8744th most frequent Russian word.
петру
"Петру" is the dative case of the name "Пётр" (Peter), meaning "to Peter" or "for Peter".
In this sentence, 'Петру' is in the dative case, which is used to indicate the indirect object of the verb (who receives the action).
Я дал книгу Петру.
I gave the book to Peter.
Here, 'Петру' is again in the dative case, used to indicate possession or belonging.
Это принадлежит Петру.
This belongs to Peter.
In this instance, 'Петру' is in the dative case because the verb 'помогать' (to help) requires an object in the dative case.