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.