If you see a fly in amber, and it's got a millimeter of amber around it, well, that could have been done yesterday or a year ago. But, if you see the fly is trapped in a huge block of amber, you know it's been there for a long time—it's been accumulating. So, a blockchain is a series of blocks. Each block is a series of computations done by computers all over the world using serious cryptography in a way that's very hard to undo.