京 (jīng) is a bound morpheme — you'll always see it in a compound word, never alone. It means 'capital city' (in words like 北京 'Beijing' and 南京 'Nanjing'), serves as an abbreviation for Beijing (in 京剧 'Peking opera'), and in classical contexts can mean ten million. Because it's a bound form, you need to pair it with another character; don't try to use it as a standalone noun.