If two bodies touch, or if a body touches a static feature in its environment, the contact is represented by one or more "contact points", described by dContactGeom.

The convention is that if body 1 is moved along the normal vector by a distance depth (or equivalently if body 2 is moved the same distance in the opposite direction) then the contact depth will be reduced to zero. This means that the normal vector points "in" to body 1.