When there is a disagreement among a crypto community hard forks result in a chain split. Transaction Replay is when a transaction is valid on both chains that have splitted from one blockchain. If you own a coin prior to its hard fork you would own an equal amount of coin on both chains after the split (coin1 and coin2). If you send coin1 after the fork, the person you sent it to can send the same amount of coin2 on the other blockchain. This is called a Replay Attack because your transactions are replayable. Replay Protection is something that makes transactions valid on one chain and invalid on the other chain. When a coin forks without replay protection, you can mix your coins with a transaction that exists on one chain but doesn’t exist on the other chain. By mixing your coins with unreplayable transactions will make your transactions unreplayable.