A friend of mine just emailed me this, and it's amazing. Since 2013 I've been finding better and better videos explaining bitcoin and blockchain technology.
This, I think, may be the best explanation of a blockchain I've ever seen. It's 17 minutes long, but it actually goes through the details of what's happening on the distributed ledger and why it's immutable and trustworthy. Even if someone doesn't understand what a hash algorithm is, this video explains things from start to finish.