The Report Explorer is an aid to navigating report elements. The Report Explorer allows you to see the structure of the report and to select any item in the report. The Report Explorer can be especially handy for complex reports where components may be nested and where it would be difficult to select an item with the mouse.
In the screenshot below you can see that the report is represented by the root node of the tree. Underneath that you see the styles, report parameters and the sections of the report and for each section/item the report items in the section/item.
The Report Explorer can be accessed from the context menu View | Report Explorer when right-clicking the area next to the report design surface.