The Model-Explorer shows the hierarchical structure of your graphical model.
Within the Model-Explorer processes are displayed as nodes. If there is a + in front of the process, you can click it to show the flows going in (-->) and out (<--) of the process. If the process is defined as a subsystem, you can also see the sub-processes
If you select an object in the Model-Explorer, it will be also be selected in the Drawing Area, and in the Property window you can see its properties and values.
If you right-click an element within the Model-Explorer, possible actions will be displayed.