The actor is A metaphysician in the dark. (19-20)
If a modern poem is an actor, and an actor is a metaphysician in the dark, then poetry is a metaphysician in the dark. That means that poetry has to get to the core of our beliefs. It has to get behind our eyes and see the world through our perspective. It needs to make us ask basic questions about how we look at the world around us and how truthful our senses actually are.
Containing the mind, below which it cannot descend, Beyond which it has no will to rise. (24-25)
It doesn't do modern poetry any good to shoot for something above the human mind (like God) or something below it (like physical objects). In the end, the modern poem has to be about the mind itself. It has to be about the mind searching for some sort of spiritual fulfillment.
The poem of the act of the mind. (28)
Well, this just about sums it up. It makes sense that Stevens makes this the final statement of the whole poem. Just remember: he's not just talking about a poem of the mind. He's talking about a poem "of the act of the mind." The difference is that he wants to talk about the process that the mind goes through when it thinks about something beautiful or something that gives us spiritual comfort. Different people like different things, but the mental processes we go through when we find meaning in the world are the same. At least for Stevens, they are.