Medi-packs instantly heal a resting hero. They are especially useful when doing skirmish battles (where you probably only have 1 or 2 good heroes). If you follow the strategy of leveling up many heroes at once, you won't need as many medi-packs as you can just move on to another hero when one dies. Medi-packs are obtained from lesser or free rally at the Scriptorum, and some daily or special challenges.