This is the 6235th most frequent Polish word.
belgradzka
"Belgradzka" refers to something related to Belgrade, the capital of Serbia.
The word 'Belgradzka' in this sentence is used as a proper noun, indicating the name of the street, derived from the adjective form of Belgrade.
Kamienica przy ulicy Belgradzka jest bardzo stara.
The townhouse on Belgradzka Street is very old.
Here, 'Belgradzkiej' is the locative form of the adjective 'Belgradzka,' used to denote the location.
Jestem absolwentem szkoły na Belgradzkiej.
I am a graduate from the school on Belgradzka street.
In this case, 'Belgradzka' is used substantively as a noun to indicate the name of the district.