Rolling resistance is the drag or friction resisting movement. In the case of tyres, it is caused mainly by non-elastic forces such as hysteris and plastic deformation. In simple terms this is the tyre absorbing some of the energy that is put into it when it deforms (under weight or impact). Rolling resistance is affected by not only the material of which the tyre is made, but also by the the combination of the air pressure in the tyre, the surface the tyre is running over and the weight put over the tyre.