In "Ode on a Grecian Urn," the speaker is speaking to the urn itself. In the last lines, "Beauty is truth, truth beauty," -that is all/Ye know on earth, and all ye need to know," the speaker says this to the frozen figures on the urn. For them, who do not exist in reality, truth is simple; it is just the beauty of the depictions on the urn. They don't have to worry about aging or change. The beauty of the scene is permanent. Someone, even a thousand years from now, can look upon the same urn and experience the self-contained beauty of the urn. Upon looking, one might imagine who these people were and since they are frozen in time, forever youthful, the scene is even more romanticized because the artifact (urn) will be admired and imagined about, potentially forever.
In real life, however, truth and beauty are elusive and subject to change. Not to mention, there is the obvious truth that beauty is subjective, open to interpretation. Physical beauty is also superficial and as we age we become less beautiful (according to whatever current notions of beauty are). For the figures on the urn, the simplicity of "beauty is truth" is appropriate; their beauty and truth do not change. For those of us in the real world, the question of truth is more complicated, as it should be. The quest for truth is an ongoing historical process.