This one seems pretty self-explanatory. After all, this is a book about Horton (our big friendly elephant) and how he hears a Who. But when you think about it, it's not really about how Horton hears the Whos, it's about how he hears the Whos when no one else does.
What makes this story special is that Horton is willing to hear and believe in something he knows is right. In a book about how little people can have a voice, too, it's especially important that someone should hear them.