This is the 156th most frequent Czech word.
petr
"**Petr**" is a Czech male given name equivalent to "Peter" in English.
Here, 'Petr' is used as a proper noun, referring to a person's name.
Petr je můj bratr.
Peter is my brother.
In this context, 'Petrova' is the possessive form of 'Petr', indicating ownership.
Kniha je Petrova.
The book is Peter's.
Here, 'Petra' is the accusative case form of 'Petr', used as the object of the verb 'saw'.