Any item that appears in the tree menu with the plus sign next to it means that the level can be expanded to show another level of pages. To expand the next level click on the plus.
Conversely when the minus sign is shown next to a level on the tree this level can be collapsed so that it only shows the parent item. To collapse the next level click on the minus.
The context menu contains all the commands available for an individual item in the tree and can be accessed by using the right click on the mouse over the item in the tree that you wish to access.