To scratch the paint job on a car with a set of keys.
One holds the heads of the keys between fingers-- pointy toothed bits out, fob and lucky charms in the palm of your hand-- and swipe at the surface of the car like you are the comic book character Wolverine. Holding your keys like this is also a good way of self-defence when pacing through an urban jungle.
Keying a car is viewed as either an act of vandalism or retribution.