This is the 674th most frequent Polish word.
lublin
"Lublin" is a city in eastern Poland.
Here, 'Lublin' functions as a proper noun representing a city's name.
Lublin to miasto we wschodniej Polsce.
Lublin is a city in eastern Poland.
The word 'Lublina' is the genitive case of 'Lublin', used here to specify inhabitants of the city.
Mieszkańcy Lublina są bardzo gościnni.
The inhabitants of Lublin are very hospitable.
In this case, 'Lublin' is used as a direct object in a past-tense sentence indicating a visit to the city.