Hi Keith - welcome to the forum!
The throttle curve changes the feel of the controller slightly. If the default is a straight line (linear) then a curve that starts under that line will give a more gentle acceleration low down on the throttle. A curve that starts above the line will give a more aggressive start. The different curves might suit different cars and different driving styles.
The best thing to do is to try the different options and see if you can notice any difference - you might not. If you get a car you are finding difficult to control (too sluggish or too punchy) then changing the curve can help - as can reducing the max power setting for a particularly punchy car.
I hope that helps.