"The Little Prince" begins with a flashback, which puts some of the themes of the narrative into a context and lends them meaning.
The narrator (the pilot) had attempted, when he was a child, to draw a boa constrictor with an elephant inside its stomach. This was due to reading about the eating habits of boa constrictors and, in his imagination, assuming that one would be able to eat an elephant. This conclusion was not something that adults were able to understand through his drawing; they thought it was a hat. The narrator then attempts to draw a cross-section, showing the elephant inside the boa constrictor, but this apparently unnerves the adults who see it, and the narrator disliked having to be so explanatory only for his enthusiasm to be met with disapproval. Thus the narrator chose not to pursue art.
This summarizes several of the themes explored in the story, such as the complexity and nuances of human relationships, the simplicity and inquisitiveness that we tend to lose when we become adults, and the unsupportive, suppressive effect that adult expectations can have upon a child. It also explains the narrator's ability to understand and interact with the Prince, and provides a means for them to relate to one another based on their mutual difficulties with expression and being understood.