To understand the metaphor one must find the tension in John Keats's "Ode on a Grecian Urn." This tension is between the aesthetic ideal and reality. The images on the urn, that of maidens and fair youths, will forever be beautiful and "For ever wilt thou love," but, as in reality, they will never have the physical pleasure of the culmination of their love.
Also on the urn is the depiction of "pipes and timbrels" that play in "wild ecstasy," but the viewer can never experience the sound of their music.
Thus the art is "desolate," for it "can ne'er return" to life. "Beauty may be truth," but it is truth in its ideal, not its reality. The urn can teach its viewer of truth, but only the person can experience real truth and actual beauty. Only in reality can one learn of passion, only in reality can one enjoy the sensual sounds of the piper.
See the question/response listed below which gives further explanations on this ode.