This is the 1666th most frequent Russian word.
петра
"Петра" is the genitive or accusative case of "Петро," a personal name equivalent to "Peter" in English.
Similarly, 'Петра' is in the genitive case of 'Пётр', used to express ownership or association ('Peter's book').
Я нашёл книгу Петра у входа.
I found Peter's book at the entrance.
Here, 'Петра' is again the genitive case of 'Пётр', indicating possession ('Peter's birthday').
День рождения Петра был вчера.
Peter's birthday was yesterday.
In this sentence, 'Петра' is the genitive case of 'Пётр', used to show possession or association ('Peter's house').