NOS means New-Old Stock. It basically refers to old parts that have never been used. In the case of keys, it's easy to tell that they've never been used, because usually they've never been cut. A great many of the keys we stock are NOS, even the aftermarket blanks. That's because many of them are either no longer made, or special order.
NOS does not mean that keys are original for your car! Keys made 10 years ago may be called NOS, even though the car they fit was made 50 years ago. Keys made by a company that never had anything to do with the manufacturer of your car could be NOS. NOS is good if you can't get original keys, but original (OEM) keys are best.