Something that is orexigenic stimulates appetite. For example, the hormone ghrelin is a chemical "hunger signal" that travels to the brain and binds with a specific receptor that triggers your appetite.
As above, orexigenic can be an adjective (an "orexigenic hormone"). It can also be a noun, as in a substance used to stimulate one's appetite. A doctor might prescribe an orexigenic to help patients maintain weight when fighting chronic illnesses or conditions that cause muscle tissue to waste away. The term comes from the Greek orexis, meaning "appetite" or "desire," combined with the suffix -genic, which means "producing." Essentially, an orexigenic is any substance that generates a physical desire to eat.