Merged Mining / Group Mining is the process of mining multiple cryptocurrencies at once.
Merged mining can only take place on networks that have the same underlying Hash Functions; SHA-256 based currencies cannot be mined with Scrypt based currencies, and so on. In the nature of blockchain, a merged mining environment exists alongside its own consensus mechanism known as AuxPoW.
Then comes the interesting point on why this methodology even exists in the first place? Essentially, new networks can bootstrap their security by leveraging other networks hash power. This has been a point of much debate amongst the developer communities because of “issues” revolving around centralization.