The 500 Hats of Bartholomew Cubbins. Two things: who and why. It's that simple.
The who is, obviously, Bartholomew Cubbins. It's a tradition started in many children's stories going all the way back to Grimms' Fairy Tales such as "Rapunzel" and "Cinderella," and one Dr. Seuss himself would continue when he titled The Lorax, The Sneetches, and this book's own sequel, Bartholomew and the Oobleck.
The why—as in "why you should read this story"—is where the 500 hats come in. Why does Bartholomew have 500 hats? What are they for, and what to do they do? To the uninitiated, the title plants these questions into their mind, setting up a mystery for what lies in the pages between that cover.