You can activate the Auto-Potion System by dragging a selected potion to the area with a black box next to your character bar. Turn it on by checking the box. Then type in a number in percentage so that it automatically heals your HP when your health drops below that number.
In the heat of combat it can get pretty difficult to keep an eye on your health. With the auto-potion system you can set certain potions to be used by your character and at what health percentage they should use those potions.
Simply drag potions from your inventory or hotkey bar to the auto-potion window in the upper left and set the percentage of health at which your character should start automatically using potions. Going against weaker monsters? Set the percentage low as to not lose potions. Going against tough boss monsters? Set the percentage high to start spamming potions earlier and not risk getting knocked out by a lucky critical!