Consider a disagreement with a scarecrow. The scarecrow will not argue back; therefore, you are free to argue about anything you want. Furthermore, you can alter its argument so that you can more easily defeat it, and it cannot object to the change. To defeat the scarecrow's argument, you can change it any way you like so that you can easily light the straw man on fire and consume him.
A straw man argument is essentially a made-up argument that doesn't really exist.