It's rather built on trustlessness, actually. :D

It mostly depends on the person to explain to and their background.

In this case, you might tell him it's a way to store and transfer money, globally, borderless, near-instantly and with negligible fees, and as such a competitor to PayPal, Western Union, and also banks (savings and checkings account), i.e. it will seriously cut into their market share. As such, of course there is serious market value behind it.

With the twist that Bitcoin is not a company. (This might be hardest thing to grasp for most out-of-band people out there.) It is a network that is run by its users for its users on the internet. For technical reasons, it uses its own tokens (small-b bitcoins) that are used as the transfer units, that are limited in amount, and are traded on various marketplaces and exchanges to and from national currencies. The bitcoin tokens also represent the shares in this network, and hence represent the value of this network measured against its mentioned competitors. Hence, this is no scam.