A shared history without a central manager
When you use a bank, the bank maintains its own transaction records. Bitcoin has no single organization responsible for keeping the only official record.
Instead, independent computers can keep and verify the transaction history using the same rules. That history is organized in the blockchain.
Transactions are grouped into blocks
A new Bitcoin transaction is first broadcast to the network. It does not immediately become part of the blockchain: it may wait until a miner includes it in a new block.
Think of a block as a group of transactions together with information that lets the network link it to the history that came before.
Why do the blocks form a chain?
Every block includes a cryptographic reference to the previous block. This gives the history an order: each newer block links back to its predecessor.
If someone alters old information, the cryptographic links no longer match. Changing the accepted history takes far more than simply editing a file.
Miners and nodes have different jobs
Miners use Proof of Work to propose new blocks. But they cannot decide by themselves which rules everyone must follow.
Full nodes independently check whether transactions and blocks follow the rules they enforce. A node can reject an invalid block even if a miner spent energy producing it.
What does this mean when you receive bitcoin?
A transaction appearing in your wallet does not necessarily mean it has been confirmed. The first confirmation arrives when it is included in a valid block on the chain your wallet follows.
As further blocks are added, the work needed to replace that history increases. That is why “unconfirmed” and “confirmed” are different statuses.