what is a Blockchain

A blockchain is a system of recording information in a way that makes it difficult or impossible to hack, cheat, or change the system. A blockchain is essentially a digital ledger of transactions that is duplicated and distributed across the entire network of computer systems running nodes on the blockchain. Any person a part of the network is entitled to and can run what they call a node.  Once a block is solved, complete, and confirmed, it is then added to the blockchain connecting to the last block that was complete before that one.  Then the next complete block is added to that block and so on.  The further back a block is on the chain, the more secure the block is.

Bitcoins blockchain is not just an ordinary blockchain.  A "blockchain" preserves both truth and lies equally.  There is a thing in blockchain called the oracle problem, whoever put the information in the blockchain, may be or may not be telling the truth.  There is always the counter party risk, unless it's Bitcoin; Bitcoin is a self contained system, the bitcoin lives within the blockchain, if you tether it to something physical then you have to trust someone, there is a counter party there.