This is the 783rd most frequent Polish word.
dobrą
The Polish word "dobrą" means "good" in its feminine accusative form.
The word 'dobrą' is used here as an adjective modifying a feminine singular noun ('książkę') in the accusative case, meaning 'good'.
Mam dobrą książkę do przeczytania.
I have a good book to read.
Here, 'dobra' is the nominative singular form of the adjective for a feminine noun ('decyzja'), meaning 'good'.
To była dobra decyzja.
That was a good decision.
In this sentence, 'dobrą' describes 'kawę', which is a feminine singular noun in the accusative case, signifying 'good'.