The throttle rate control setting on an RC car's ESC essentially lets you tone down the car's power when accelerating, so you don't lose traction and control.
If your car tends to lose grip when accelerating, a common problem on low-grip tracks or for new drivers, you may want to turn this setting down to help you keep from spinning the wheels.
If your throttle control (referring to you, the driver, not the ESC) is pretty good, you may not need this setting, and can leave it at its maximum value.
See our article on electronic speed controller for more info on ESC settings.