This is the 214th most frequent Polish word.
rosja
"Rosja" means "Russia" in English.
Here 'Rosja' is used as a proper noun denoting the name of the country Russia.
Rosja to największy kraj na świecie.
Russia is the largest country in the world.
'Rosja' represents the nation in a historical context, providing a reference to its past political state.
Rosja była kiedyś częścią Związku Radzieckiego.
Russia was once part of the Soviet Union.
In this question, 'Rosję' is the accusative case of 'Rosja', showing its syntactic role as the direct object of the verb 'odwiedziłeś' which means 'have visited'.