What does cross-chain messaging mean?

Today, you will often hear the term ‘cross-chain messaging’ but mainly what it is referring to are ways for one blockchain to communicate with another blockchain. There are many ecosystems today with Ethereum and its layer-2 roadmap, layer-0 protocols such as Cosmos/Polkadot/Avalanche, and other layer-1s such as Solana. There are many reasons why a single blockchain cannot infinitely scale, so a multichain world will exist and they will all need to have some way to communicate.

Of course, there are many implementations of how to do this communication - all of which have varying security, composability and scalability...