XCM, or Cross-Consensus Messaging, is a language developed by Polkadot for communication between different blockchains (parachains), and even with external chains. It enables a variety of interactions including asset transfers, smart contract calls, and data sharing across different blockchain systems. This protocol is key to enabling interoperability within the Polkadot network and beyond, fostering a scalable multi-chain architecture where different blockchains can work together seamlessly and securely.