I have read of other people getting customers/orders before actually having the product in place. But my doomsday mind has a hard time shaking what happens if

I have read of other people getting customers/orders before actually having the product in place. But my doomsday mind has a hard time shaking what happens if you can't follow through? Is it as simple as refunding the money? A simple, stupid question with probably a single answer but that is where my mind goes when I hear people doing things this way.