The Diplomacy pane allows you to interact with other characters throughout the game. It is accessed by clicking the ‘scroll’ along the top left set of buttons, or by clicking the same scroll on any character’s pane (below their portrait to the left).
The Diplomacy pane shows a range of interactions that are available between yourself and the character you have selected. If you have selected yourself, there aren’t a lot of options.
With another ruler selected, far more options are available.
The options available to you will vary based on the circumstances at the time. For instance, the ‘Call to Arms’ option at the bottom of the list above is only available to you when conducting diplomacy with an ally during a time of war.