The poem of the mind in the act of finding What will suffice. (1-2)
If this is a definition of modern poetry, then it seems like poetry's job is to show us how a human mind can find some sort of spiritual fulfillment in life. As Stevens goes on to add, people haven't always had to look for meaning as hard as they do now. But the modern world has made it more difficult for people to believe in religion and endless scientific progress, so poetry will have to pick up the slack in the spirituality department.
It has to be living, to learn the speech of the place. (7)
If poetry is going to offer people spiritual fulfillment, it can't go on talking in the language of Shakespeare. It has to learn the language of regular folks at a specific historical moment and learn how to speak to their deepest desires. In other words, poetry shouldn't try to be timeless anymore. It should try to speak to people in the here and now.
It has to think about war And it has to find what will suffice. (9-10)
Poetry can't just be all love and roses. The horrors of the modern age have made people too smart for that. Poetry needs to acknowledge all of the ugly things in life (like war) and it has to find a way of making life seem worthwhile in spite of these terrible things.