The blockchain is a decentralized, distributed, cryptographically secured, immutable database. The blockchain provides a peer-to-peer mechanism to find consensus about the valid state of the system, i.e. if a transaction is accepted as valid and unique by all or should be rejected. The blockchain manages any kind of transactions in a secure and transparent way without the need of a centralized trusted authority.
Initially built to support digital assets like Bitcoin, the blockchain is extremely resilient to disturbances and robust against hostile actors: Even if somebody manages to corrupt the consensus forming process, i.e. to arbitrarily change what is accepted as the next entry to write into the data base, the rest of the network will recognize the attempt and be able to stop it. Thus, the blockchain is the best technology to facilitate safe machine-to-machine interactions.