A replay attack happens when a malicious actor intercepts and then repeats a valid data transmission that goes through a network. This is possible because the attacker has gained access to valid credentials for the network. Because of this, network security protocols treat the attack as if it were normal data transmission. In this way, the attacker can carry out his attack without major problems.
Replay Attacks especially affect technology blockchain, due to its operation as a distributed system. This is because precisely this operating structure facilitates this type of malicious action. All you need is to have access to valid credentials and that the attack is carried out at the right time, with it enough to achieve success.