An App Origin object, or formally an App::OriginGroupExtension, is a class supplying selectable elements that represent the three standard axes (X, Y, Z) and three standard planes (XY, XZ and YZ) to objects that are intended to arrange different types of geometry in space.
Std Part (App Part) objects and 
PartDesign Body objects are created with Origin objects by default. If needed, Origin objects can be added to 
Assembly objects of the 
Assembly3 workbench, too.
Left: The tree view showing three objects that use Origin objects. Right: Representation of the Origin elements in the 3D view.
The axes and planes are objects of type App::Line and App::Plane respectively. Each of these elements can be hidden and unhidden individually with the Space bar. This can be useful when selecting the correct reference for creating other objects e.g. Sketches.
Simplified diagram of the relationships between the core objects in the program. Two of them have an Origin object to control the placement of the objects grouped under them.
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