This pretty neat. One way to view this is that they are kind of like lightning channels but fully trusted - settlement is delayed indefinitely and your counterparty can rug you. But, you get to choose how much money you trust them with explicitly.
Cassis is a way to leverage (and enhance) social trust in order to send satoshis without incurring in complicated and costly blockchain operations.
You may ask what is the point of all this: "just trusting friends with money? Isn't that trivial? Couldn't we have done that already? And if we didn't doesn't that mean it is a bad idea? What innovation is Cassis bringing up to the table?"
I'm not sure how useful this is in its current form, but it makes me wonder if we might find a useful non-custodial protocol that ratchets up trust a bit.