How does a blockchain actually work?

A blockchain is basically a chain of data blocks, where each block stores info about transactions and is cryptographically linked to the previous one - if someone tries to change old data, the chain doesn't match up anymore. That makes it super hard to manipulate the data because you'd basically have to change all the blocks that come after it too. That's why people use it for cryptocurrencies and other stuff where trust and transparency matter.

What a lot of people don't get: the security doesn't work because individual blocks are super heavily encrypted, but because the whole chain is connected. The second you change an old block, the hash no longer matches the next block, and that becomes immediately obvious. That's why you'd basically have to recalculate the entire chain from that point on - and with huge networks like Bitcoin, that's practically impossible because way too many other computers would notice right away.