Actually, I have also been with people with some knowledge of blockchain, so I never thought about how to explain blockchain to a newbie.
But if that happens; I will tell them that blockchain is like a group of ledger (or transactions) that are connected with each other to form an interconnection or a chain, in which a particular transaction is linked to another. In this case, you cannot undo a particular transaction that has been initiated because they are immutably connected to other transactions.
Okay I don't know if I'm making sense here :)