What is the exposition of the lorax?

The exposition of "The Lorax" introduces the Once-ler, who tells the story of how he came to cut down all the Truffula trees and the consequences that followed, leading to the desolation of the environment and the departure of the creatures that once thrived there. This sets the stage for the rest of the story to unfold as he reflects on his actions and the importance of taking care of the environment.