Strip away the hype, and a blockchain is a fairly simple idea: a shared record of transactions, copied across thousands of independent computers instead of stored on one company's server. Each new batch of transactions is grouped into a "block", and each block is cryptographically linked to the one before it — hence the name.
That link is the important part. Because each block depends on the exact contents of the one before it, changing an old transaction would require redoing every block that came after it, on a majority of the network, simultaneously. In practice, on an established network, that's not realistic — which is what makes the history so difficult to quietly rewrite.
But a shared ledger alone doesn't solve the hardest problem: how do thousands of strangers, who don't trust each other, agree on what the "correct" version of that ledger actually is? That's the job of a consensus mechanism. Bitcoin uses proof of work, where computers compete to solve a computationally expensive puzzle to earn the right to add the next block. Many newer networks, including Ethereum, use proof of stake instead, where validators lock up (stake) the network's own asset as a financial commitment to behaving honestly, and can lose it if they don't.
Here's the part worth correcting early: "it's on the blockchain" doesn't automatically mean something is trustworthy, anonymous, or safe. A blockchain can only guarantee that a recorded transaction happened and hasn't been altered — it says nothing about whether the thing you're interacting with (a token, a smart contract, a project) is legitimate. That judgement is still entirely down to you.
Understanding this one idea — a shared, tamper-resistant record maintained by many independent parties, with no central authority required — is genuinely the key that unlocks almost everything else in crypto, from how Bitcoin works to how DeFi protocols operate.