What does it mean to be a naturalized football player?

Players who voluntarily acquire the nationality of other countries other than the nationality of their country of birth. According to the query of Baidu Encyclopedia, naturalized players refer to players who voluntarily and actively acquire the nationality of other countries other than their birth nationality, and are essentially skilled immigrants. For example, Kaman of the German team, Gordon of the British team, blache, Dorset and Javier of the Philippines.