Addons are objects that are instantiated by Eclipse 4's dependency injection framework. Addons are global and are contained under the application.
These addon objects are created before the rendering engine actually renders the model. As such, addons can be used to alter the user interface that is produced by the rendering engine. For example, the min/max addon that comes with the Eclipse 4.x SDK tweaks the tab folders created for MPartStacks to have min/max buttons in the corner.