http://y7yea4pmqqtznb33qiugvysyn2bob5v62e4pvoadoibrwkq3tsddjeyd.onion/glossary/man-in-the-middle-attack
When you first connected to Bahram, you actually connected to this person, and she, in turn, connected to Bahram. When you think you are asking Bahram a question, she receives your message, relays the question to Bahram, receives his answer back, and then sends it to you. Even though you think you are communicating securely with Bahram, you are, in fact, only communicating securely with the spy, who is also communicating securely to Bahram!