Krishna signals a new segment of his teaching. He tells Arjuna: now hear the threefold division of intellect and fortitude, classified by the gunas. I will explain each one separately and completely.
The structure of this section is remarkably orderly. Knowledge, action, doer — those have been covered. Now come intellect (the faculty that decides right from wrong) and fortitude (the inner strength that holds mind, breath, and senses steady). After these, happiness will follow. It is a complete map of the human inner world, sorted by sattva, rajas, and tamas.
By addressing Arjuna as Dhananjaya — conqueror of wealth — Krishna subtly reminds him: you have already conquered external battles. Now conquer the inner ones. Understanding your own intellect and fortitude is the way.