Something that's admonitory is meant to correct or scold. If you're caught throwing paper airplanes in class, your teacher will probably give you an admonitory lecture.
Use the adjective admonitory to describe something that's done in a warning or reproachful way. When kids are loud in the library, the librarian might shush them in an admonitory way. If your piano teacher always finds fault with your playing, she is consistently admonitory. The root of admonitory is the verb admonish, which means "to scold or reprimand." The Latin origin of both is admonere, "remind or suggest," but also "warn or urge."