Effect updates and description changes can be seen here.    

    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.        

This card is also very good in combination with Froggit Trio, as an alternative for Final Froggit. (Or combo with Final Froggit if you have enough money.) It also is an amazing counter to Toriel.

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.