Throughout "Ode on a Grecian Urn," the speaker describes the images with which the eponymous urn is decorated. In pondering these images the speaker also reflects upon the nature and purpose of art.
In the final lines of the poem, the speaker indicates that the images on the urn seem to communicate one idea, namely that "beauty is truth, truth beauty." In other words, the speaker reflects that beauty and truth are one and the same thing. Perhaps we might take this to mean that wherever and whenever one finds beauty, one will also find truth in that beauty. For example, the beauty of a person might point to the pleasing truth of that person's character. Likewise the beauty of the natural world might point to what many Romantics like Keats considered a truth—that God manifests himself through nature.
We might also say that there is a truth in the beauty of art, such as the images with which the urn is decorated. The beauty of art perhaps points to the truth that human beings are inherently creative, imaginative beings.
We could also postulate that wherever there is truth, there is also, necessarily, beauty. There is often a simplicity to truth that appears beautiful, especially when there is, surrounding that truth, so much that is untrue, disingenuous, and deceptive. There is also a beauty in the truth, or realization, that humans are creative beings capable of producing timeless, beautiful works of art, such as the eponymous urn.