This is the 9673rd most frequent Polish word.
zabrzu
"Zabrzu" is the locative case of "Zabrze," a city in Poland, used to indicate a location ("in Zabrze").
Here, 'Zabrzu' is the locative case of 'Zabrze', indicating the location, used with the preposition 'w'.
Mój dziadek mieszka w Zabrzu.
My grandfather lives in Zabrze.
Again, 'Zabrzu' is in the locative case, used because of the preposition 'w' indicating the place.
Mam rodzinę w Zabrzu, którą odwiedzam co roku.
I have family in Zabrze that I visit every year.
In this third sentence, 'Zabrzu' is used similarly in the locative case to specify the location 'Zabrze' with 'w'.