For a high-level introduction, this article in The Economist is where I'd advise...

For a high-level introduction, this article in The Economist is where I'd advise someone to start: http://www.economist.com/bitcoinexplained

Then, to make things feel more grounded, watch this video of a maths teacher walking you through the mechanics of what actually happens. He starts with "four friends who sometimes spot each other $5 for lunch" and then builds from there with great visuals, so it is quite accessible. https://www.youtube.com/watch?v=bBC-nXj3Ng4