Friction while roller skating comes from at least a couple of sources
The movement of the wheels on the surface being skated on. The amount of friction will be determined by what the wheels are made of ( rubber, plastic, wood, etc) and the surface being skated on (sidewalk concrete, polished concrete, wood, composite, etc)
Another source of friction would be the connection between the wheels and the axles. The wheels could be fixed to axles that are inserted in a slot. This would allow the axle/wheel to turn in the slot. Then there would be friction between the slot in the axle. I think most skates now have axles that are fixed and do not spin. The wheels have ball bearings so that the wheel can spin on the axle. Friction would occur between the inner surface of the wheel and the ball bearings and between the ball bearings and the axle. Some type of lubricant is used in the ball bearing assembly to minimize friction. Still, the different materials of the wheels, axles, ball bearings and lubricants produce different amounts of friction.