All of us understand how screws and bolts work. So imagine if you encountered a screw that you could advance--but not retract. I.e. you can screw it in, but it won't unscrew…unless you turn it from the other side. If you're confused by what I mean, watch this "impossible screw" video and see if you can figure out what the hell is going on, before he reveals the secret:
I can't think of any practical applications, beyond bringing this to a bar and using it to trick people into buying you drinks.