Hero Aid is used to increase hp by a certain percent depending on the hero level used and if the hero is a rare or epic it also gives you an aid skill. For example if you use a level 20 Archane Warlock(rare) for a hero aid on blitz bomber(epic) you give blitz bomber a 5% hp increase and archane warlock increases sharpshooter mercanery hp by 2% at level 1 aid skill. You can increase your heroes aid skill by leveling there active skill for example for the archane warlock we used if you increase her storm salve to level 2 it will increase her aid skill to level 2 as well. You unlock the hero aid in the hero hall at hero hall level 3 and you unlock an additional hero aid at level 5. That means if all five of your hero halls are level 3 you can use 5 hero aids and at level 5 they can use up to 10 hero aids. You can get to your hero aid by clicking select hero in the hero hall then click hero aid.