Replay Protection Definition
An implemented replay protection means that an update disables same transaction from being displayed on each blockchain. Without replay protection replay attacks are possible during the first blocks post-fork, since both chains will have same private keys after the fork.
A replay protection matters most when cryptocurrencies are forked. You’ve probably heard “replay protection” in relation to the November 18th SegWit2x Bitcoin fork. The 2x coin will not have replay protection (rather, an opt-in protection), Bitcoin Cash did have it.