Acceleration is the rate of change in velocity. When you first think of acceleration, you think of speeding up, but, as the definition states, slowing down is also acceleration. Roller coasters use specific amounts of acceleration to get the perfect thrill for that particular ride. The more acceleration, generally, the more thrill.
Friction is the force resisting the relative lateral motion of two solid surfaces in contact. In other words, friction is the resistance of two objects in contact from moving in a lateral motion. If you try to slide down a slope of sandpaper, you wont get very far (plus the fact of the pain you would be in). If you try to go down that same slope, but instead of sandpaper, there was ice, it would be much easier to do. Roller coasters have to compensate speed with friction because without speed, there is no movement.