This is the 2071st most frequent Polish word.
moja
"Moja" means "my" or "mine" (feminine form).
In this sentence, 'moja' is a possessive pronoun indicating that the book belongs to the speaker. It is in the nominative feminine singular form because it is used with 'książka,' which is a feminine singular noun in nominative case.
To jest moja książka.
This is my book.
'Moja' here is also a possessive pronoun, modifying 'mama' (mom), which is a feminine noun in nominative singular form, indicating possession by the speaker.
Moja mama gotuje obiad.
My mom is cooking dinner.
'Moja' describes possession and is used with 'restauracja' (restaurant), which is feminine singular in nominative case.