WARNING: AS OF SEASON 42, THIS CARD HAS BEEN REPLACED BY DOUBLE TROUBLE.
Effect updates and description changes can be seen here.
Aaron's Secret is a Bravery Deck specific effect card. It gives +1 ATK to 3 random monsters on your board. 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 its ability.
Keep in mind that you are better off with using a Strength on a 0-1 ATK monster, as this card won't increase its ATK enough to become immune to Muffet's Pet's effect.