Man-in-the-middle attack (MITM)

In the context of cryptography, authentication means "ensuring that the message you got comes from who you think it did".

Authentication is how we prevent the man-in-the-middle attack.

Authentication is one of the hardest parts of cryptography, because the only truly secure way to do it is by driving to the other party yourself to establish a pre-shared key so you can do message authentication code. Or to share your public key with them if you are satisfied with the safety of post-quantum cryptography.