Someone cuts you off in traffic. You feel the heat in your chest. You say to yourself, I am angry. The sentence has two parts. There's an "I." There's an anger. The "I" has the anger. The "I" is over here, watching the anger over there.
K's challenge: look for the "I" watching the anger. Try right now. Find it. What you find is more anger. The "I" you imagined was a separate observer is itself the anger, narrating itself. The watcher is the watched. The thing observing the heat in your chest is the heat in your chest, plus a label.
This is the move K is most famous for. It collapses the dualism people walk around inside without noticing. The structure of "I am angry" assumes a subject and an object. Me, and my anger. K says the subject is the object. There's no separate "me" that has anger. The "me" is the anger, momentarily packaged with a pronoun.
When the move lands, something changes in how anger runs. Normally there's a fight. "I" versus "anger." "I" trying to suppress "anger." "I" losing or winning. With the move, there's no fight. No two sides. Just the anger, doing what anger does, with no opponent. Strangely, it tends to subside faster without an opponent than it did with one.
I am angry is grammatically fine. As a description of what's happening, it's a lie. There's no "I" next to the anger. There's just anger, calling itself an "I."