Alice and Bob think they are talking directly to each other. Instead, they are both talking to Eve (the "man" in the middle). Alice and Eve do DH exchange and

Alice and Bob think they are talking directly to each other. Instead, they are both talking to Eve (the "man" in the middle). Alice and Eve do DH exchange and agree upon a secret. Eve and Bob do a DH exchange and also agree upon a secret. Now Eve knows both these secrets and can relay messages between Alice and Bob. Both think they are talking securely to each other, but they are actually talking to Eve instead