A blockchain is a decentralized digital ledger. It’s decentralized because there is no single entity in charge of maintaining the ledger. Many parties, called validators, maintain their own copy of the ledger. The validators use a consensus protocol to ensure they always agree on the contents of the ledger.
A desirable property of blockchains is that validators agree on the contents of the ledger even in the presence of byzantine (malicious) validators who do not follow the rules of the consensus protocol.