This is the 12335th most frequent Polish word.
moją
"Moją" means "my" (used for feminine singular nouns in the accusative case).
In this sentence, 'moją' is used as a possessive pronoun in the singular form, referring to 'my' and modifying the singular noun 'książka', which is feminine.
To jest moja książka.
This is my book.
Here, 'moją' is the accusative form of the possessive pronoun in feminine singular, used to indicate the ownership of 'przyjaciółkę' (a singular feminine noun referring to 'friend').
Wczoraj spotkałem moją przyjaciółkę.
Yesterday, I met my (female) friend.
In this case, 'moją' again takes the accusative form to align with the feminine singular noun 'mamę', showing possession and relational context.