Final answer:
Activators in developer solutions initiate the conversion of exposed silver halides to metallic silver.
Step-by-step explanation:
In the context of developer solution used in photography, an activator is a substance that initiates the development process by converting the exposed silver halides into metallic silver. The most common activator used in developer solutions is a reducing agent, such as hydroquinone or phenidone. These compounds provide the necessary activation energy to facilitate the conversion of the exposed silver halides into visible black metallic silver grains.