A “Replay Attack” can happen when two forked crypto-currencies allow transactions to be validated across both chains.
For example, if Bitcoin experiences a fork and is split into different versions like BT1 and BT2, each Bitcoin holder will retain his holdings and receives an equal amount of the forked cryptocurrency.
The address, private key, algorithm and transaction format remain the same on every chain. When you transfer BTC1, your BTC2 can be accidentally transferred away too. This is called “Replay Attack”.