A man-in-the-middle attack (MITM attack) as the name suggests occurs when a hacker positions themselves between two parties, trying to intercept their communication. It is only through modifying the traffic they are able to retrieve the information.
MITM attack often occurs via re-routing the traffic to a phishing website that is created to impersonate the legitimate one. It becomes very difficult to actually detect such attacks. By avoiding mutual authentication, a hacker can impersonate both ends of the communication.
In the case of blockchain, the majority of the cryptographic protocols use endpoint authentication to prevent MITM attacks. A mutually trusted certificate of trust can also be used to prevent such attacks. This is where the MITM attack further evolves into a meet-in-the-middle attack.