This is the 4075th most frequent Russian word.
новосибирск
Novosibirsk: A city in Russia, the administrative center of Novosibirsk Oblast and the largest city in Siberia.
The word "Новосибирск" is used here as a proper noun, specifically as the name of a city in Russia.
Новосибирск — это третий по величине город России.
Novosibirsk is the third largest city in Russia.
In this sentence, "Новосибирске" is the locative form of the proper noun."
Я был в Новосибирске прошлым летом и увидел множество достопримечательностей.
I was in Novosibirsk last summer and saw many sights.
Here, "Новосибирск" serves as the subject of the sentence and points to the city as characterized by its notable features.