What is the force slows down a falling skydiver?

Air friction slows down the skydiver. If he has a parachute, he increases his surface area so that the air around him has more "space" in which to push against his free fall. Without the parachute it's the same thing just with less resistance to the air therefore he falls faster.

Friction

is the force resisting the relative motion of solid surfaces, fluid layers and material elements sliding against each other