Gizmos (also called the modifier apparatus) are viewport icons that provide a visual representation in the scene of a modifier at that the user can manipulate. They can be as simple as the 3D box used by the bend modifier, or as complex as an FFD lattice.
For simple modifiers, a gizmo is a 3d box that the modifier has been applied to. For more information see Simple Modifiers.
A modifier plug-in must implement a method to display the modifier's gizmo in the 3D viewport. This is done by implementing a member function of the BaseObject class called BaseObject::Display(). The GraphicsWindow class provides a number of drawing routines for this purpose.
If the plug-in is inherited from SimpleMod, the member function SimpleMod::Display() provides a default implementation that draws the gizmo as a 3D box to which the modifier is applied. For example, the Bend modifier's gizmo is drawn as a bent 3D box. The developers may display their modifier's gizmo in any manner they wish, if they are not inherited from SimpleMod.