I tell them that it's digital money, the currency of the future, liberated from the inflationary banking system.
That all transactions are public and that the balance of each address is the sum of it's transactions. That the blockchain is decentralized, replicated and very safe. Transactions are irreversible. Seize of account and blocking are impossible.
Bitcoin will still take a lot of value because there will be only 21M of Bitcoin. The gold rush is only at it's beginning.
There are always more uses: Bitcoin can be bought from ATMs in swiss railway stations and we can pay bills to the city of Zug for example.