To be fair, most cryptocurrency
is nonsense sci-fi bullshit. I hesitate to even call it money.
Ok, let me try ELI5ing it.
Let's say we have an economy where, in order to determine who owns what, people literally just make and publish a list of who owns what. Everyone trusts the lists. But what if someone decide to change the list so that it says they own someone else's stuff? Proof of Stake says that if someone wants to change the list - say, if someone traded something on the list to someone else - the page they add to the list also has to have some of their money taped to it. If that change turned out to be fraudulent, someone else can tattle on them, and then that someone gets the money taped to the list instead. So if you want to make sure you're reading from a not-fraudulent list, just check for the money taped to the page.
This all continues fine for a while. There's a bunch of people who maintain the list, all of whom have their own money taped to different pages of it.
One day, someone tries adding a fake transaction to the list. You, of course, notice it and tattle on them. Except in order to claim the funds taped to the list, someone else has to... add a note to the list saying you own them. And there's no reason why they can't just write down that they tattled instead of you, because the only proof that you were the one who tattled is getting it onto the list that you have to tape money to in order to add to. So the only people who have a reason to make sure the list is valid are the same people who are updating the list in the first place. Everyone else can tattle but they don't get paid for doing so, which was the the thing keeping the system secure.