皇 = 'royal/imperial' (from 皇家 huángjiā 'royal') and 马 = first character of 马德里 (Mǎdélǐ 'Madrid'). So 皇马 = 'Royal Madrid' compressed into two characters — exactly how Chinese sports media abbreviates club names (e.g. 巴萨 Bāsà = Barça).
One of the most successful football clubs in history, founded in 1902. The Chinese fan base is huge — players like Cristiano Ronaldo (C罗) and Sergio Ramos (拉莫斯) became household names through 皇马.
