Smoothness is an important component of track driving. What is it exactly though?
My current definition is smoothness is the speed with which driver inputs are applied. For example, "smooth steering" refers to slowly turning the steering wheel, and "smooth braking" means pressing the brake pedal slowly.
"Slowly" in this context is relative, often to what the driver is presently doing. The way I communicate this to drivers is when I want the drivers to be smoother, I say something like "turn the steering wheel slower".
On a more advanced level, smoothness refers to changes in driver inputs. For example, in steering, smoothness is how quickly the driver transitions from no steering angle to a steering angle at which the car is turning. Once this transition is made, steering angle can be added at the same rate. While advanced drivers may appear to be abrupt, what typically happens is advanced drivers start the inputs slower and gently accelerate the inputs until the full desired input is achieved, whereas novice drivers start the inputs quickly and decelerate the inputs once their hands and feet are in motion.