We don't know how Keats personally saw the urn, but we know what he says in the poem about it. (They may be different.) In the poem about it, he indicates that art, and those thoughts, feelings, and actions portrayed in art, especially great art, are made immortal. Daily life is always stressful, in part because we know we're going to die, and will pass. Great art, though, lasts, and in this poem, it freezes those perfect moments forever, and the urn symbolizes that.
Greg