If you look at all of Stevens' poetry, you'll find that he's a pretty big fan of this sort of title. Some of his other poems include "Of the Surface of Things," "Of Heaven Considered as a Tomb," and "Of Hartford in a Purple Light." The "Of" is a fancy way of saying that Stevens' is about to deliver a short talk about whatever subject comes after the word "Of." It's plain and straightforward, no nonsense—"Here's what I think modern poetry should be, and that's that."