A blockchain is essentially a digital ledger that stores records of transactions, duplicated and distributed across a network of computer systems. Each block in the chain accommodates several transactions. Once a transaction is added or validated, every block in the network adds the transaction into their ledger.
If one of the blocks in the network has been compromised due to a malicious transaction, the blockchain will immediately know that one of the blocks has been tampered with and hence can be terminated. To successfully hack the network, the attackers need to change every block across the distributed versions of the chain. Therefore, it is almost impossible to hack an entire blockchain.
Bitcoin and Ethereum are some of the most famous blockchain networks in the world. The addition of new blocks in these systems adds up the security of the blockchain.