What does the "Attic shape" represent in "Ode on a Grecian Urn" by John Keats?
The term "Attic shape" in the final stanza is a synonym for the urn itself. "Attica" is Greece, and "Attic" means relating to Greece or Athens; therefore, "Attic shape" is a parallel construction for "Grecian Urn," which appears in the title. The poet begins the poem by addressing the urn itself as "thou," calling it a "bride," "foster-child," and "historian" in the first stanza. In stanzas 2, 3, and 4, the poet describes and wonders about the figures and events depicted on the urn. In the final stanza, he returns to addressing the urn as a whole again, even personifying the object, imagining that it speaks in words at the end of the poem. Using the term "Attic" plays on the word "attitude," used three words later, because of the similar sound of the two words. "Attitude" here means the pose that an artist places something in while it is being captured, but it can also mean one's viewpoint or perspective toward an issue or toward life itself. By the end of the ode, the Attic shape has shaped the poet's attitude, so much so that the poet senses the urn influencing his understanding of beauty, truth, and what is really important in life.
Keats begins the fifth stanza by addressing the urn as an "attic shape." The word "attic" refers to Attica, a region of ancient Greece of which Athens was the chief (primate) city. "Shape" simply refers to the shape of the urn. Therefore, the speaker of the poem is saying that the urn was crafted in Attica.
If you are searching for symbolism or some other relevant literary device (since you ask what attic shape "respresent[s]", I do not believe that it is present here. I believe that this is just a brief mention of the city in which the urn was made.