I borrow your car (and you even give me the title!), promise to return it to you (but not necessarily the same car, just the same make and model), and then I sell what is truly now my car to someone else. That new owner could then find someone else to lend the car to, transfer the title on a promise that they'll eventually transfer the title back, and then let the new borrower sell it, transfer the title, etc. There's only ever one title and one car, but there are a lot of promises to return the car back later. When you explain it with goods it becomes obvious that a fraud was perpetrated if the buyers don't realize that the car might be owed to someone else. Yay for financialization and long impenetrable terms of service agreements with your broker where you automatically allow your broker to "lend" and re-title your shares so that they can earn interest on the lending.

This is a simplification though, there's actually like a parking garage involved (broker) who says to trade on his exchange that the broker will keep your title safe for you - it's better than a paper certificate to hold in your safe at home because it can't get lost! But this allows the parking attendant to sell your car hoping you wont notice, and hoping that he'll be able to buy another similar car back before you actually ask for yours back. And of course insurance companies, auto dealerships, etc, but you get the idea.

Robinhood's genius is hiding this complexity from their users behind a slick "gambling is fun" style app. TD Schwab ETrade and other "adult" brokerages also don't make it obvious, but at least they make you "read" some documents that explain the details before you get an account.