This is the 7475th most frequent Polish word.
kosowo
"Kosowo" in Polish refers to "Kosovo," a region and partially recognized country in Southeast Europe.
Here, 'Kosowo' is used as a proper noun referring to the name of a specific country.
Kosowo to państwo na Bałkanach.
Kosovo is a country in the Balkans.
In this sentence, 'Kosowa' is the genitive case of the noun 'Kosowo', indicating a relationship of origin (from Kosovo).
Mój wujek wrócił właśnie z Kosowa.
My uncle has just returned from Kosovo.
Here, 'Kosowo' is used as the subject of the sentence, a proper noun used in the nominative case.