LSD or Limited Slip Differential.
In a non LSD vehicle, under acceleration the power will go through the diff to whatever has the least resistance which under normal driving is both wheels.
In poor conditions, or gravel or say if you break traction (ie burnout) then the wheel that has the least traction has the least resistance, so the power will go through the diff to that wheel.
With an LSD it will vary or even lock all together the difference in rotation of the 2 wheels, so on take off if you have less traction in 1 wheel then the majority of power will still be going through to the other wheel.
This is good for take offs, but you may notice instead of you wheels spinning, you might find your car pulling in a different direction than usual.
Depending on the LSD you have chosen, you may also find it corners differently as you are changing the rotational design of the car (1 wheel usually spins faster than the other when you corner) and in a worst case scenario this can result in a car hopping around a corner.
Thats why LSDs come in many different types depending on your requirements.