This is the 1413th most frequent Portuguese word.
artur
"Artur" is a Portuguese male given name, equivalent to "Arthur" in English.
Here, 'Artur' is used as a proper noun, specifically as a person's name, indicating the subject of the sentence.
Artur é um amigo muito confiável.
Artur is a very reliable friend.
In this usage, 'Artur' indicates possession, referring to the owner of the book that was mentioned in the sentence.
O livro de Artur foi perdido.
Artur's book was lost.
Here, 'Artur' is the object of the verb 'encontraram,' showing that the named person was the one encountered by them.