This is the 2593rd most frequent Polish word.
słoweniec
Slovenian (a person from Slovenia).
Here, the word "Słoweniec" is used as a noun referring to a person from Slovenia.
Słoweniec pochodzi z małego kraju w Europie.
A Slovenian comes from a small country in Europe.
In this sentence, "Słoweniec" is used in the instrumental case "Słoweńcem" to describe my friend being of Slovenian origin.
Mój znajomy, który jest Słoweńcem, podróżuje po Polsce.
My friend, who is Slovenian, is traveling through Poland.
The word "Słoweniec" appears here in the instrumental case as "Słoweńcem" because it's part of a constructed phrase expressing action towards someone.