This is the 8798th most frequent Polish word.
russia
The Polish word "Rosja" translates to "Russia," the country, in English.
Here, 'Rosja' is used as a proper noun to name the country 'Russia'.
Rosja jest największym krajem na świecie.
Russia is the largest country in the world.
In this case, 'Rosji' is the genitive form of 'Rosja', used in association with the noun 'historia' (history).
Myślę o historii Rosji.
I am thinking about the history of Russia.
Here, 'Rosji' is used in the locative case indicating the destination 'to Russia'.