Aaron's Secret is a Bravery Deck specific effect card. It gives +2 ATK to all your monster that have 3 or less ATK. This is useful on cards such as Ice Cap and Temmie. You can place these down, use this and then leash out the damage. It can also be useful to prevent Muffet's Pet from activating it's ability.
Keep in mind that you are better off with using a Strength on a 0 ATK monster, as this card won't increase it's ATK enough to become immune to Muffet's Pet's effect.