Replay attack (also known as playback exploit) is a form of network exploit in which a valid data transmission is maliciously or fraudulently repeated or delayed.
Replay attack is carried out either by the originator or by an Attacker who intercepts the data and re-transmits it, possibly as part of a masquerade attack by IP packet substitution.
Suppose Alice wants to prove her identity to Bob. Bob requests her password as proof of identity, which Alice dutifully provides (possibly after some transformation like a hash function); meanwhile, Eve is eavesdropping on the conversation and keeps the password (or the hash). After the interchange is over, Eve (posing as Alice) connects to Bob; when asked for a proof of identity, Eve sends Alice's password (or hash) read from the last session, which Bob accepts thus granting access to Eve.