The Upkeep Monitor is the area of the screen that shows icons for all the enchantments that you are currently maintaining at an upkeep cost of -1 energy regeneration each. You can end a maintained enchantment by double-clicking its icon in the Upkeep Monitor.
The Upkeep Monitor will tell you the name of the skill and the name of the affected player, if you hover your cursor on it.
By default (if you haven't customized your interface) the Upkeep Monitor is located right above the Skill Bar.