Bitcoin can be best describe in various ways depending on your audience but a general introduction would be, 'Bitcoin is decentralized money/currency'.  Then you can go on to explaining what you meant by decentralized but they should already get the concept of what you meant since the world decentralized is quite easy to understand. You can then go on to lecture them on how centralized the traditional fiat is and give examples on how your asset can be frozen by the government fie their selfish reason but such can't be possible on the Bitcoin network.

Bitcoin is first a currency before an investment, you should always make this clear to your audience because Bitcoin seems to be losing its currency features and been turned fully into an investment vehicle, which isn't entirely bad but I think that's defeating the main idea behind the creation of the currency in the first place. Satoshi needed a system that'll be full proof from centralized operation and to become a global currency for business operations, Bitcoin is yet to achieve that but it's already been seen as an investment vehicle globally.