I wouldn't tell much but will use simple words such as its a digital currency that can be use as payment for goods and services online whereas the seller is willing to receive bitcoin.
It has an unstable value that the market gives depending on the demand and supply of it. And if that isn't enough its better for him/her to watch tutorials that's explanation bitcoin with some visual graphics.
It is good to tell the basic first for them to realize how bitcoin work and the benefits of it that can give. I have to tell that not only can invest on bitcoin but also it can used for paying goods and services and this can give change to everyones life. You have to adopt the goodness of having knowledge about bitcoin because you can able to earn it.