In "Ode on a Grecian Urn," the speaker begins by describing the urn itself as a "still unravish'd bride of quietness." Essentially, the speaker is referring to how the images decorating the sides of the urn are frozen in time, never to be resolved.
This static quality is presented as double-edged. On one hand, the images depict an idealized world of beauty, most powerfully in the image of a young lover in pursuit of his beloved. The speaker observes that the young woman in the image will never have to worry about losing her beauty, since she will never have to grow old. Even death will be a foreign concept to these young lovers as they remain in the springtime of their lives for as long as the urn exists.
However, the images depict moments of anticipation without any resolution: the men will always chase the group of women without either catching or losing them, the lover will never get to kiss his beloved, and the heifer will never reach the place of sacrifice. The comparison of these scenarios to a bride whose marriage has not been consummated is an apt one, since these images all depict a static innocence—and hence a mild sense of frustration.