Exponential settings control the response curve of the steering or throttle, respectively. They do not affect the maximum steering angle or maximum throttle/braking - those adjustments should be done with end-point adjustments or dual-rate adjustments.
With a 0 exponential steering setting, your steering servo will respond linearly as you turn the transmitter wheel, that is, if you turn the wheel 15%, the wheels on your car will turn 15%.
With a negative expo setting, the initial steering response is lessened, making fine steering adjustments easier, but as you turn the transmitter wheel further towards the endpoints, the response speeds up. For instance, if you turn the transmitter wheel 15%, the steering servo may only turn 10%, but then as you turn farther, the wheels will "catch up", so they're turned 100% when your transmitter wheel is 100%. This can be useful on open, high-speed tracks where you want a lot of control around steering center, but need less fine-grained control around the extremes.
With a positive expo setting, initial steering response is more twitchy but then becomes more gradual towards the endpoints. This may be more useful on tight tracks where you need to make a lot of quick turns.
With a 0 exponential throttle setting, your throttle will respond linearly as you pull the trigger on the transmitter, that is, if you pull the trigger 15%, the servo will turn 15%.
With a negative expo setting, the initial throttle response is lessened, making fine low-end throttle adjustments easier, especially useful on low-grip or tight tracks. As you pull the trigger back farther, the throttle responds more quickly.
With a positive expo setting, initial throttle response is more immediate but then becomes more gradual towards full throttle.