What Is “Merge Mining” and How Does It Enhance Security for Smaller Coins?

Merge mining allows two different cryptocurrencies, using the same Proof-of-Work algorithm, to be mined simultaneously by the same miner without any loss of efficiency. The smaller coin "piggybacks" on the hashrate of the larger coin.

This means that to attack the smaller coin, an attacker would need to acquire a majority of the combined hashrate, which is typically the hashrate of the much larger, more secure parent chain. This makes the smaller coin vastly more expensive to attack.