天王 has multiple layers of meaning. In mythology, it refers to a 'heavenly king' or deity in Buddhism and Daoism (e.g., 四大天王 'Four Heavenly Kings'). Historically, it was used as a title for emperors, most famously Hong Xiuquan, the leader of the Taiping Rebellion, who called himself 天王 ('Heavenly King'). In modern pop culture, especially Chinese-speaking regions, 天王 describes a top-tier celebrity or the absolute best in a field — think of the 'Four Heavenly Kings of Cantopop' (四大天王). The specific sense depends heavily on context.