You'll probably get a lot of different answers to this question, but I'll start. Emerson called for a "new" American literature, a literature worthy of the scope, size, and newness of the American country. Whitman responded to this request. He threw aside the conventions of earlier poetry and gave us a poetry worth of our democracy, a poetry that attempted to encompass the immense variety of people/nationalities/races that contributed to the American of Whitman's time. Whitman also brought an almost mystical vision to his writing, seeing everything as a part of a great whole, everything somehow joined to everything else, and, in his almost endless lists, where things just appear in no particular order, everything equal to everything else.
Emerson recognized Whitman's greatness, and in a famous letter celebrated him: "I greet you at the beginning of a great career, which yet must have had a long foreground somewhere, for such a start. I rubbed my eyes a little, to see if this sunbeam were no illusion; but the solid sense of the book is a sober certainty. It has the best merits, namely, of fortifying and encouraging."
A good place to start studying Whitman is his "Democratic Vistas" essay. E-notes provides a good summary at the source below.
Hope you enjoy your study of Whitman!