A blockchain is a system of recording information in a way that makes it difficult or impossible to hack, cheat, or alter the system. Essentially, a blockchain is a digital ledger of transactions that is duplicated and distributed across the entire network of computer systems running nodes on the blockchain. Anyone who is part of the network can run what is called a node. Once a block is solved, completed, and confirmed, it is added to the blockchain, connecting to the last block that was completed before it. The next completed block is then added to that block, and so on. The further back a block is on the chain, the more secure it becomes.
Bitcoin's blockchain is not just an ordinary blockchain. While a "blockchain" preserves both true and false information equally, there is a concept in blockchain technology called the oracle problem. This refers to the fact that whoever inputs information into the blockchain may or may not be telling the truth. There is always a counterparty risk, except in the case of Bitcoin. Bitcoin's blockchain is essentially a network of truth and integrity. Any information added to the blockchain is visible to all participants in the network, who can then confirm and verify the block. There is no counterparty risk because the network is constantly being verified by its participants, regardless of how much Bitcoin they hold or if they hold any at all. This is because Bitcoin uses proof of work rather than proof of stake.