To put it simply, a Blockchain is a form of a public ledger on which anyboody can see all the transactions with that crypto coin.
How does it work:
A Blockchain works by using a consensus mechanism to place correct transactions onto a block where it is verified by other nodes in what is called consensus.
When all nodes have verified that all transactions placed on the block are correct and not fraudulent, the block that is added to the blockchain is now available for anyone to see the transactions that have been made and added to that block.
To put it simply in short, the consensus mechanism is the way that the code rewards validators with coins for placing the correct transactions onto a block and allowing for the the Blockchain to function.
This consensus mechanism can be in the form of a Proof of work consensus mechanism (Bitcoin, Ethereum 1.0); or in the form of a Proof of stake consensus mechanism (solana, Ethereum 2.0, cadano)