This isn't a long poem, so Stevens doesn't waste any time getting down to it. Modern poetry, he says, has to be "The poem of the mind in the act of finding/ What will suffice" (1-2). In case that sounds a little obscure, Stevens spends the rest of the poem explaining it.
In short, Stevens starts by saying that poets used to have an easier job. They knew what was good (i.e., classic poetry) and they did their best to imitate it. But the twentieth century brought along some changes that made it a lot harder for people to enjoy poems about courtly lovers and humble shepherds; so poetry had to change, too.
Next, Stevens takes ten or so lines to explain just how poetry is supposed to adapt to the changing times. Rule #1: It has to be flexible. Rule #2: It has to talk about things that matter to normal people. Rule #3: It must be willing to talk about the ugly parts of life just as much as the pretty ones.
Finally, Stevens says that modern poetry has to have a single subject at its core, and that subject is poetry itself. Modern poetry has to be about the act of writing poetry. More specifically, it has to be about making people feel better about their lives. In Stevens' words, it has to be "the finding of a satisfaction" (26). It's up to the poet to decide what that satisfaction is, but in dark, modern times, poetry has to give its readers a sense of hope and purpose. For Stevens, the modern world's already a tough enough place to live. It doesn't need poetry to make it any tougher. (You taking any notes, T.S. Eliot?)