what is a clay bar?

A clay bar is like a lint brush for your car. The idea is that it's sticky enough to pull off little bits of debris (dead bugs, road tar, tiny pieces of gravel) but not so sticky that it would pull off your car's finish. Despite its name, clay bar is usually made of plastic resin. You don't have to clay your car more than a couple of times a year. Basically, you wash the car, wet the clay with water or clay lubricant, then run the clay over the car and it will pick up little sticky things (tree sap, bugs, bird droppings) that the washing didn't get. Then you follow up with wax or polish. Yes, you can definitely use it on a car that has been clearcoated.