Look at the rock sitting on the hill in the picture above. gravity should make the rock slide down the hill. what force is acting to balance gravity, keeping the rock in place?

I believe the answer you are looking for is Static Friction. Static Friction is the force that holds an object in place until it starts to move. Then it switches to rolling friction.

For example, if you have a 1/2 ton truck sitting in front of you and the truck is in neutral. (meaning it can roll if pushed). The truck is extremely hard to move at first. That is because static friction is holding it in place until the amount of force exceeds the limit of static friction.

So if we continue to push at the truck and you feel it starting to move, then once it starts moving it is much easier to push, that is because we moved past static friction to rolling friction. Rolling friction is what helps slow things down. If you roll a ball across a carpet floor it eventually comes to a stop.