This is the 6532nd most frequent Swedish word.
mattias
"Mattias" is a Swedish male given name, equivalent to "Matthias" or "Matthew" in English.
'Mattias' is used here as a proper noun, referring to a person's name, and it's the subject of the sentence.
Mattias gick till affären.
Mattias went to the store.
'Mattias' in this sentence is used as a possessive noun, indicating ownership of the car.
Det är Mattias bil.
That is Mattias' car.
'Mattias' is again used as a proper noun, indicating a participant in the activity and functioning as part of the compound subject of the sentence.