I am doing a company talk (we do them weekly) about network security and at least the way I was planning to explain tailscale to our staff was via a picture I still need to somehow build, but it goes something like this:

You have a crowd of people (like one of those massive crowds you see in Tokyo etc). And you are both at either sides of the crowd, and you each have a can with a string connecting you to each other – and I explain that the crowd is the public internet, and tailscale allows you to create that string across the whole crowd in a way that nobody in the crowd can ever listen to you guys talk, even though the string literally passes through/over tens of people in that crowd.

If someone has a photo like that, I'd gladly use it :)