Brake strength adjusts the amount of trigger brake the ESC will apply to the motor. There are 1-13 steps available using the onboard QuickTune system, with each step being about 8% brake strength. You can also fine tune the brake strength via the HotWire at a higher resolution of adjustment, with 0-100% steps available.
Brakes are extremely important and you need to adjust them appropriately. Track surface, moisture content, tires and car will all come into play when setting brakes. Typically a good brake strength is going to be 80% or less. I find myself setting even as far down as 55% at times, which really helps me not over brake and carry more speed.