This is the 8650th most frequent Polish word.
płocku
"Płocku" is the locative case of "Płock," referring to the city of Płock in Poland.
The word 'Płocku' here is used as the locative case of the proper noun 'Płock', indicating the location where someone lives.
Mieszkam w Płocku od pięciu lat.
I have been living in Płock for five years.
The word 'Płocku' functions in the locative case to specify the place of an event, combining with the preposition 'w'.
Spotkałam go kiedyś w Płocku na rynku.
I once met him in Płock at the market square.
In this sentence, 'Płocku' identifies the location in the locative case for the question, coupled with the preposition 'w'.