What did Harold Rosenberg mean...?

What he was saying is that a painting is no longer JUST a painting. It has become like a play which is being acted out. He uses the ogy of the canvas being the arena or stage and the concepts in the painting as characters for the painter to act out their own show. He is saying that the painting have come to life and they personify the emotions that they are portraying. Instead of just being a picture of an object, mundanely reproduced, it is now an entire play that is created on this metaphorical stage.