The action Bar shows the current commands that you have assigned.
To assign an action to your action bar open the desired panel (Commands Panel or Emote Panel) select you desired action, drag and drop it into the desired slot.
To execute an action, Right Click it, or press the shortcurt key displayed above the icon.
You can move the action to another slot by dragging it and dropping in the desired slot, however this will overwrite any existing action in that slot.
To remove an action from the slot, simply Left Click it.