The rigid body is an assemblies of one or more physical entities. The assembly is rigid and dynamic.
Rigid bodies give the dynamics properties to the entities that are inside the body. In other words, if an entity is not a child of a rigid body, it will be static (not subject to the laws of physics). Static geometries do not react to force, but are involved with collision detection. A dynamic entity is therefore subject to force propagated by contact with another static entity.
In the simulator, the rigid body defines the mass center (inertia matrix) of the rigid assembly composed by the geometries.
The mass center and global mass value is automatically computed by the simulator according to geometries properties. In this case, the rigid body's position in the editor is ignored by the simulator that will recompute the good position at simulation startup.
In this case, the rigid body's position in the editor is the mass center. The global mass value must be filled.
In a world, you can use static geometries (physics entities that are not connected to a rigid body) to create infinite masses entities that do not consume CPU. For example, a building, bridge, a wall or fence. In general, these entities do not need to be dynamic, it is therefore unnecessary for the solver to lose time to make a balance of forces on these objects.
Since Marilou 4.3.1, the rigid bodies can become static (Kinematic property): the assembly is rigid (you can select and move it from Exec view) but is considered as static by the simulator (does not consumes any CPU time, and does not accumulate any numerical errors).