Imagine that you're locked in a room. You don't know any Chinese, but you have a huge instruction book written in English that tells you exactly how to respond to Chinese writing. Someone outside the room slides you a piece of paper with Chinese writing on it. You can't understand it, but you can look up the characters in your book and follow the instructions to write a response.

You slide your response back out to the person waiting outside. From their perspective, it seems like you understand Chinese because you're providing accurate responses, but actually, you don't understand a word. You're just following instructions in the book.

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.

https://en.wikipedia.org/wiki/Chinese_room