Just for fun, I tried to explain capabilities using only the ten hundred most used words. Here’s what I came up with.
They are a way to allow people to use the things they are allowed to use and not the things they are not allowed to use by giving them a key for each thing they are allowed to use. Every thing has its own key. If you are shown a key you can make another key for the same thing. Keys should be very, very, very hard to guess.
For capability purists: yes, I am describing network capabilities.