Anandamide receptors. They don't only respond to weed- cannabis happens to be the only plant we know of that produces large amounts of THC or CBD. Anandamide is a chemical our body makes to stimulate those receptors "endogenously" (IE, for its own inner working). The purpose of anandamide receptors is still quite cryptic, or rather, I would say fairly subtle- it seems to "enhance the experience of pleasure in response to reward".