Most axles come with an open differential from the factory, which means that the wheels can spin at different speeds to make the vehicle handle better around turns on pavement where the wheels spin in different arcs and thus travel different lengths in a turn. The problem is that when you take it off-road, only one tire has to lose traction for the other to spin, because the force goes to the tire with the least resistance, which would be the one spinning. A locker replaces the open differential, and when locked it sends 50% of the power to each wheel, traction or no traction. Off-road this means that if one of your wheels is spinning, the wheel with traction will still continue to push you forward. There's more to it, and there are more than one type of locker, but that's a summary of the basics.