"Blockchains allow me to send something of value to you over the web without any third party being involved".

Thats my explanation of Blockchain. Then I go on to use the "Cash" example (No banks, No Paypal) and people then get the concept.

Finish up with telling people as the law catches up with Blockchain technology and you can get other things of value onto Blockchains other than just cash we could sell houses or used cars directly to each other, instantly, with no lawyers or notaty's and see the past history of that property or car over it's entire life from the day it was first sold.

No need to explain the tech, just explain the "WOW" factor.