is the"dummy" means a ..human body model?
No, it's not a crash test dummy or something...
It's called "dummy", because it's not a real (i.e. complete) Object3D. It has no polygons, no vertices and no properties (well, it has them but there are ignored). It doesn't have to be added to an instance of World either. All it really is, is a kind of container for a transformation matrix. This is due to the fact that jPCT is not a pure scene-graph based API but borrows some elements from the scene-graph concept.
Anyway, what is it good for? Ok, imagine a planet spinning around the sun and a moon spinning around the planet. The planet itself is spinning around its vertical axis too. So the basic transformation the planet does is the rotation around its own center while the moon should rotate around the planet. Two problems arise here: The moon should follow the planet if it's moving (i.e. spinning around the sun, which i'll cover later) but it can't be a child of it, because it must not inherit the planet's rotation around its center. The other problem is, that the planet is spinning around itself, but not around the sun. A dummy object is a convienient way to solve this. You create a dummy object and place it into the center of the planet. This dummy object has its rotation pivot in the center of the sun. This way, the dummy rotates around the sun, which is what the planet should do to. So you attach the planet as a child of the dummy. The same goes for the moon: Set the rotation pivot to the center of the planet and attach it as a child to the dummy. This way, both objects will follow the dummy around the sun while performing their own, independent rotations too.
Have a look at the downloadable jPCT-demo. There is a room with a planet and a moon rotating around a pillar that illustrate exactly (almost...) this effect.
Somewhat clearer now?