On an RC car's electronic speed controller (ESC), the brake control setting determines how make braking power is applied. It reduces the amount of braking power through the brake travel, making it easier to maintain control of the car and less likely that you'll lock up the wheels.
This is different from the max brake force which essentially limits the maximum force of the braking power. Brake control, on the other hand, reduces the braking power sent to the motor over the range of braking, thereby giving a more mild braking response that is less likely to make the car shift its weight suddenly and lose control.
Experienced drivers may want to use the full braking power and not limit it at all, or they may only use it on low grip tracks where full braking power would be more likely to disrupt the car's balance.
See our article on electronic speed controller for more info on ESC settings.