When there is a disagreement among a crypto community hard forks result in chain split. Transaction Replay is when a transaction is valid on both chains. For example, if you hold a coin before its hard fork you will own an equal amount of coin on both chains after the split. If you send your coins after its fork, the person you sent it to can send the same amount of coin 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.