Without replay protection, new transactions will be equally valid on both chains as well. This means that these transactions can be copied or replayed, from one chain to the other, in other words, for them to happen on both which is commonly referred as replay attack. If you send B2X to someone, you might accidentally send him BTC as well and vice versa because they can do a replay attack with the help of a miner in both chains.
http://www.nasdaq.com/article/segwit2x-and-the-case-for-strong-replay-protection-and-why-its-controversial-cm849792Without replay protection, new transactions will be equally valid on both chains as well. This means that these transactions can be copied or replayed, from one chain to the other in other words, for them to happen on both. This is called a replay attack.