The Outliner is a list of everything that is the current Set, Scene or Shot that you are working on. To open the Outliner Tab, click on the tab in the upper left corner of the software (Quadrant 1). (Hotkey - Alt+8)
In the following example of an outliner of Set:
...
Close - this will close the panel (Hotkey - Alt+8)
Functions of the Outliner
...