Man In The Middle

Wiki: https://en.wikipedia.org/wiki/Man-in-the-middle_attack

Attacker secretly relays and possibly alters the communications between two parties who believe that they are directly communicating with each other, as the attacker has inserted themselves between the two parties

Even when using RSA or ECDSA as user authentication algorithm, the MITM can actually impersonate both client and server using different public-private keys. Thus middleman decrypt all the traffic going between client and server. (see this video on key-exchange problem)
Read the signature and authentication to see how MITM impersonation is avoided.
If an MITM can not impersonate client/server, its difficult for the middleman to decrypt the messages.

But, the middleman can alter the bits of the message. To avoid this, Message Authentication Codes like HMAC or AEAD are sent along with the message which determine the integrity of the message, thus detecting any errors.

In any case, the middleman can always hinder the communication between 2 parties, such that none of the messages is ever validated due to errors.