Friction is the force that two surfaces exert on each other when they rub. Smooth surfaces produce less friction than rough surfaces. Without friction a moving object might never stop moving.
Example:
When a person uses a sled to sled down a snowy hill, the sled is able to move quickly down the hill because there is very little friction between the hill and the sled. If a person tried to slide down the hill without a sled, friction is what prevents them from gaining speed.
Example:
When a marble rolls down a ramp and onto a carpeted floor, the floor and the marble exert a friction force on each other. Over time, the friction force slows the marble down and brings it to a stop.
