That’s a somewhat abstract way of saying it, so more concretely we could say that:
The blockchain is database of verified public timestamps.
Every bitcoin transaction is kept in a public ledger, and that ledger is verified and maintained by all of the computers participating in the Bitcoin network. This “chain” of transactions is known as the blockchain, and each transaction is essentially a public timestamp that can contain data.
The key aspects of the blockchain’s timestamps are: decentralized (no one entity controls the database of timestamps, and everyone in the network confirms that timestamp has happened — this is “mining”), immutable (once a timestamp has been verified and recorded, you can’t un-do it), public (all of the timestamps are publicly visible, though some aspects of the data are encrypted), and programmable (you can write code against the blockchain — for example, triggering some sort of action based on the details of a “smart contract” embedded in a timestamp).
Importantly, each of those timestamps contains a packet of data which can hold lots of things: details about a financial transaction, details about a contract between two or more parties, a hashed version of almost any document, etc.
One way I’ve described this is similar to the way people used to use postmarked envelopes to verify that something had happened at a certain time. For example, signing a will and putting it in an envelope, and mailing it to yourself — the post office’s postmark on the envelope, which has the date of the stamp, proves that whatever was put in the envelope was done so before the date of the stamp. IIRC, more than once, a mystery on Colombo was resolved using this technique, where a witness dramatically unseals a postmarked envelope before the judge.
The blockchain is essentially a digital, public, programmable version of that. Which we’ve never had before.
Previously, every app kept its own notion of time. So if I post something on Facebook, Facebook saves that post and timestamps it. We have to trust them to get that right, and not to change it ever in the future. This is fine for cat photos, but less fine for a financial transaction, or a deed to a house.
Here’s that same idea in diagram form:
So in some sense, the Blockchain is a public database — it has the effect of moving data that was previously kept within the walls of one or more apps out into a shared public database. But to get a little more specific, it’s really a public database of timestamps — a new ability for anyone to state, publicly and immutably, that a certain thing happened at a certain time.
Maybe that is obvious to folks who have been working in this space for a while, but I found it to be a really helpful way of thinking about things — and of explaining it to people who are new to the idea. Thanks Muneeb!