Rigging mode is where you add mesh regions and bones to your character. This is also the mode where you paint or auto apply weighting to bones and their associated mesh regions for skinning.
There are 3 main modes available during rigging. They are: Region, Bone and Test.
In Region mode, the user can add mesh regions created in Meshing mode to the character. This is also the mode where added regions can be transformed and manipulated.
In Bone mode, bones can be created in a hierarchical manner for the creature. This mode also allows the manipulation of added bones as well as weighting bone influences with respect to the mesh regions in the scene.
Test mode allows the user to take the rig out for a test run. In this mode, the user can see how the creature's mesh regions deform via the dragging, rotating and moving of the creature's bones. It is highly recommended that a rigged creature is thoroughly tested with different bone poses first before moving onto animation.