Imagine a notebook in which many people record transactions at the same time. Each new page (a block) contains a fingerprint of the previous one, like a seal that fits only one specific piece of content.
Why it cannot be cheated
If someone changes even a single letter on an old page, its fingerprint no longer matches the next one. The entire rest of the chain immediately stops adding up. For a swap to go unnoticed, you would have to recompute all the following blocks and convince the majority of the network to accept it at the same moment.
- Hash: a digital fingerprint of data. The smallest change produces a completely different fingerprint.
- Block: a batch of transactions carrying the fingerprint of the previous block.
- Consensus: the rules by which the network agrees on which record is the true one.
Who guards the notebook
No single party. And that is the point. Many independent computers keep a copy. To falsify the history, you would need to take control of most of them at once, which in a large network is extremely expensive.
A blockchain is not "secure" because an advertisement says so. It is resilient, because rewriting the past takes more work than it is worth.
That is all there is to it. Everything else (cryptocurrencies, smart contracts, NFTs) is built on top of this one simple idea: a shared notebook that cannot be quietly rewritten.