The ability text of some cards includes activation abilities. To use such an ability, declare that you are using it during your Main Phase while you have the card in your Domain. Activation abilities are formatted as [Activation Ability](Activation Condition): Effect.
Example: You are backing Scholar Princess Bergamotte. This gives you access to her activation type ability, which reads: “(Discard a non-Curse card from your hand): Take an Action Card from your discard pile and put it in your hand. You can use this ability once during the Main Phase of each of your turns.” You declare you’re using it, which lets you discard an Apprentice Maid card from your hand to take a discarded Alchemist and add it to your hand.