Blockchain is a distributed ledger that is immutable. It is a ledger in the sense that it is a global accounting system that keeps track of balance (and other data) of each account. It is distributed as it is not maintained by a single entity but rather by a distributed network of independent machines who are given the incentive to do so. It is immutable in the sense that it is not easy to change the chronological history of changes made to the ledger.
Blockchains have some key properties that make them a powerful technology:
Since the ledger is maintained by a decentralized network of machines that is open for anyone to join, it cannot be shutdown making blockchains resistant to any external control and censorship.
Due to the public nature of blockchains, anyone (with sufficient resources) can join the network to maintain the ledger and can make use of the network. There is no need to get permission from any central entity making it open for anyone and everyone.
The entire chronological history is public for anyone to review. This provides transparency, trust and auditability allowing anyone to re-create the current state of the system from the historical data.