The rough spots on one surface mesh with the rough spots on the other. Material is ripped from each surface, and there is a constant chattering between the two materials. Static friction . Two surfaces are in contact, but are not moving against each other. Two surfaces are in contact and are moving against each other.
Static friction is greater than kinetic friction.
As the sled is moving, inertia is overcome. There is kinetic friction, but kinetic friction is much less than static friction. It is easier to keep a sled moving than it is to get it in motion. While the friction of two surfaces can be measured, the friction can be increased or decreased by increasing or decreasing the weight or pressure between the two. If you drag a file cabinet across the floor, the friction will be reduced if the drawers are removed to reduce the pressure between the two sliding surfaces.