One (but not both) of the members of such a pair of Bodies can be a Ground. The other member Body of such a pair then has its motion defined relative to a fixed ground point. This fixed ground point does not have to be the same as the World origin. A system can have many such Ground-Body pairs and must have at least one.
How to Connect a Joint Between Two Bodies
You represent relative motion of bodies with respect to one another by connecting their Body blocks with Joints. You can connect a Body to one or more Joints.
A Joint block is always connected to a specific point on the Body on either side of the Joint. The specific point for anchoring a Joint on a Body is the origin of a Body CS, and a Joint is therefore connected on one side to one Body at a Body CS origin, and on the other side to the other Body at a Body CS origin.
Usually a Body is connected to a Joint on either side, so the default you saw earlier in this tutorial for Body CSs in the Body dialog box is three Body CSs: the CS at the center of gravity (CG) and two other CSs (CS1 and CS2). But a Body at the end of a Body — Joint — ... — Body chain is connected to only one Joint.
Choosing a Revolute Joint for the Simple Pendulum
In spite of the complexity of the concepts implicit in a Joint, the actual configuration of a Joint block is fairly simple. Here you insert and configure one revolute Joint block between the Ground and Body blocks you've already put into the model window.
A Simple Pendulum Connected to Ground by a Revolute