You always do a great job with these.
My approach:
Hey Matt, what's this blockchain stuff all about?
Things can be permanent on the internet now, and the implications of that are fascinating to me. Records of sending money to each other was first, that's bitcoin; but anything you do online has, or will soon have, a permanent alternative. Even the guy who runs the website can't undo things you do. They're permanent.

Then if they want to understand how it works;
The information is stored on identical databases all around the world. None of them change anything unless they all agree; so even if one goes rogue or blacks out, your information is safe, and nobody can pretend to be you as long as you're the only one who knows the big string of letters and numbers you got when you started.