It's pretty simple really.
Normally, the relationship between the throttle and brake will be linear. I.e. When you move the stick/trigger you get the exact same response on the ESC.
Move the trigger 5%, the esc will respond with 5%.
Now for some situations, the initial response can be too much for the grip level, causing the wheels to spin up (under throttle) or lock up (under brake). So you can use the curves to smooth out the response. Using a little bit of negative curve (the curve starts off flatter before getting steeper), means that for 5% throttle, you may only get 2% on the ESC.
It can really help to smooth the response of the car out, and can be a useful tool to help maintain traction around the centre of the throttle stick.
Personally, I tend to prefer to tune this setting on the transmitter (if it's available) rather than the ESC, as it allows more rapid adjustment on the rostrum. If you haven't got that option on the tranny, than by all means use it on the ESC.
Also, don't rely on it too much... training your throttle finger is part of the skill for going fast