First, Shelley says that the two main forces in poetry are reason and imagination. Reason is necessary to understand relationships among things in the world and imagination is important to give these ideas of relationships color.
Poetry needs harmony to balance the reason and imagination.
The idea of harmony can be seen in a number of Shelley's own works. The two of these working together is harmony. The sky-lark's song is Shelley's idea of a good poem, in that it is harmonious to the rest of the world. In it, the persona comes to the realization that nature outlasts art. The order of the world favors nature here.