This is the 8189th most frequent Polish word.
olsztyn
Olsztyn is a city in northern Poland.
In this sentence, 'Olsztyn' is used as the proper noun referring to the city of Olsztyn in Poland.
Olsztyn to piękne miasto na Mazurach.
Olsztyn is a beautiful city in Masuria.
Here, 'Olsztyn' is utilized in the locative case to indicate the location where the speaker was.
Byłem w Olsztynie na wakacjach w zeszłym roku.
I was in Olsztyn on vacation last year.
In this context, 'Olsztyn' appears in the accusative case as the target destination of returning.