Complex sentence: It is a contraction of "Atrebates", a Belgic tribe of Gaul and Britain, successively evolving through the years to become "Atrades", "Atradis", "Aras" and finally "Arras".
Split into simpler sentences: It is a contraction of Atrebates, a Belgic tribe of Gaul and Britain. Through the years the name "Atrebates" successively evolved to become "Atrades", "Atradis", "Aras" and finally "Arras".