Speed Sensitivity (my interpretation):
0% = disabled
100% = fully enabled
At 0%, it means that if you turn your wheel lock-to-lock, your car's tires will also turn lock to lock *regardless of your speed*.
At 100%, it means as you drive faster, your car's tires will turn less and less (i.e. reduces twitching at higher speed). In other words, your steering inputs are *speed sensitive*. The faster you drive, the less it turns.
If you also enable 900 degrees in your wheel setting (for wheels that support it; eg DFGT, G25, G27), then this setting is less relevant since you have full motion, but it still makes a difference. On my Xbox 360 wheel with only 270 degrees lock-to-lock, the Speed Sensitivity setting helps a lot.