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What is a blockchain?
A shared ledger that many computers keep in sync, so no single operator can quietly rewrite history.
A blockchain is a chain of batches of transactions. Each batch (a block) points at the one before it. Change an old block and every later block stops matching. That is the simple idea behind immutability.
Thousands of independent computers keep a copy. They agree on the next block through a public set of rules. Bitcoin and Dogecoin use proof of work: miners spend energy to propose the next block, and the network checks that the work is valid.
Because the ledger is public, you can look up a payment without calling a help desk. Because it is replicated, taking down one server does not erase the history.
Blockchains are slow compared with a private database. That is the trade: you give up some speed to get a record nobody owns alone.