Kick-up describes the angle between the main part of your RC car's chassis and the front suspension pins, as shown above for off-road cars (or many other types including dirt oval.) The kick-up angle is the sum of the kick-up angle of the chassis (how much it bends upwards in the front) and the angle of the front suspension pivots with respect to the chassis.
For on-road cars, the chassis usually has no kick-up of its own, so the kick-up angle is simply the angle between the suspension pins and the chassis. Some on-road cars actually have negative kick-up which is called "anti-dive" (not kick-down!)
Most RC cars offer some option to change the kick-up angle.
To learn about the affects of kick-up on your car's handling, see out our article on caster.