"Replay" attacks are "Man in the middle" attacks that involve intercepting data packets and replaying them, that is, resending them as is (with no decryption) to the receiving server.
As a result, depending on the context, the hacker can benefit from the user's rights. Imagine a scenario in which a client sends an encrypted user name and password to a server to log in. If a hacker intercepts the communication (using monitoring software) and replays the sequence, he will obtain the same rights as the user. If the system enables password modification, he could even replace it with another, depriving the user of his access.