Negative camber is when the tops of the tires are closer to the frame than the bottoms.

Negative camber is what you want. The amount of negative camber (typically -1 to -3 degrees) will vary with the application and the available traction. The more negative camber, the greater the chance of the wheels sliding and the less of a chance of traction rolling. You want to use more negative camber on higher grip surfaces for increased stability.

Basically, when you corner, weight is shifted to the outside tire and the suspension is compressed. As the suspension is compressed and the vehicle leans, it typically reduces negative camber (Making the tire more close to striaght up). If you do not have enough negative camber, the angle generated (by the corner and suspension compression) can go past vertical. If it does, in many cases the tire will roll sideways under its own (& the vehicle's) weight, then bite and flip the truck.

The idea is to have enough negative camber so that, at the most extreme corner, the tire is vertical or very near to vertical (Still with negative camber).