Just to give a simple explanation, this one may work. Bitcoin is a digital or electronic money that is decentralized in nature, meaning not controlled by the bank or government. It is used for buying, selling or even investing, and everything will be online transactions.
That's a good explanation, but don't forget to tell the negative outcomes in Bitcoin, sometimes telling them the cons and pros will enlighten their mind on how to manage their own fund. Something like this, yes, you control your own funds with a private key, but if you lose it, you lose access; if someone else gets it, they can spend it without a trace.
It's also important for them to know that it’s pseudo-anonymous, transactions are visible on the public blockchain, but the real-world identities behind addresses aren’t necessarily known.
IMO, I will not encourage people directly about Bitcoin, I want them to weigh its pros and cons on it. If they like it, for sure the rest they will find a way how to learn more about Bitcoin.