Healing an avatar is synonymous with gaining life. Healing a minion removes damage from that minion. When 'you heal X,' you increase your life by that amount, up to your avatar's maximum life. For example, Divine Healing increases your current life by 7 (to a maximum of 20 or whatever your avatar's maximum life is). If you are at death's door, your life total can't change, so any healing effect will fail. Setting life doesn't count as healing (e.g. Twist of Fate). But, if you're at death's door, your life total can't change and will remain at 0.