Its a thought experiment involving a room where people write letters and shove them under the door of the Chinese kid's dorm room. He doesn't understand what's in the letters so he just forwards the mail randomly to his Russian and Indian neighbours who sometimes react angrily or happily depending on the content. Over time the Chinese kid learns which symbols make the Russian happy and which symbols make the Indian kid happy, and so forwards the mail correspondingly until he starts dating and gets a girlfriend that tells him that people really shouldn't be shoving mail under his door, and he shouldn't be forwarding mail he doesnt understand for free.