You have discovered esoteric theories, scraps of forbidden lore that you can use to augment your magical abilities.
At 2nd level, you gain one esoteric theory of your choice. When you gain certain apothecary levels, you gain additional theories of your choice as shown in the Theories Known column of the apothecary table. Additionally, when you gain a level in this class, you can choose one of the theories you know and replace it with another theory that you could learn at that level.