That’s how I explain Bitcoin to people who ask. 

I say something like: “Bitcoin is a free bank in cyberspace that operates with its own currency. There is no one in charge of the bank but anyone can buy the currency and anyone can use it. Bitcoin can’t be hacked and the rules of the bank can’t be changed so it’s secure and fair for everyone who chooses to use it. There is a finite amount of Bitcoin so everyone’s share of the money supply always stays proportional to the total supply.” 

It works pretty well and they usually have some follow up questions from that starting point.