to say "bitcoin is cryptocurrency that mined using SHA-256 algorithm" will not work very well
if you want to explain it very simple i would say:
Bitcoin is money and a new invention. You can sent any amount to to anyone on the planet, instant and for free without asking anyone for permission. You can sent money/bitcoin as easy as an email.
That was not possible before the invention of bitcoin.
With that you can cover the money aspect of bitcoin which is enough for your little small talk