In Joint Mode you can view Joints, i.e. bones that make up character skeleton.
A character in Joint Mode
Joints make up an essential part of the rig: they are used to deform character meshes in accordance with the positions of the elements of the rig. Each joint corresponds to a particular part of a mesh (or several meshes) which moves along with it.
Joint mode can be used for editing poses, but doing so is not advised as Point Controller mode offers more convenient means for this.