At the right hand side of the editor window is an optional area which provides one of a range of useful lists of parts of your session. It is not shown by default when you first start using Ardour. The Editor list can be hidden or shown using View > Show Editor List. The very right-hand side of the list gives a selection of tabs which are used to choose the list to view. The left-hand border of the list can be dragged to vary the width of the list.