Krishna announces the structure of what comes next. Knowledge, action, and doer — each one is of three types, divided by the three gunas: sattva, rajas, and tamas. He tells Arjuna: listen to this enumeration as it truly is.
This is the opening of one of the Gita's most systematic passages. Like a skilled cartographer drawing a map, Krishna will chart the entire landscape of human behavior through the lens of the three gunas. Every kind of knowing, every kind of doing, every kind of doer will be placed in its category.
The value of this teaching is self-awareness. When you can recognize which guna is driving your knowledge, your actions, and your personality at any given moment, you gain the power to choose differently.