The Rock Dodge is a sharp, short turn used for avoiding a rock or other small obstacle you might encounter on the trail.
The idea is to avoid a small obstacle with your front and back tire while tackling varied technical terrain. To accomplish this, allow your front wheel to pass the obstacle on one side, and then turn sharply so your rear wheel passes on the other side of the obstacle.
This is a low speed skill and is most often used while climbing single track. This is a great skill to practice as it really gets you to hone in on precise movements and control of your bike.
The best place to practice this skills is on a flat, relatively open area. Once you get proficient on flat ground, progress to an up-sloping area then eventually to a climbing, technical single track.