Augustus was very interested in Greek art, and in showing how civilized Rome was by taking Greek art styles and developing them into a new, sophisticated Roman style.
The frieze shows Marcus Agrippa (on the left, with his head covered), leading a procession to dedicate the Altar of Peace (Augustus is on the other side). In many ways, this frieze is like the Parthenon frieze of four hundred years earlier, which also shows a procession.
For more about the Ara Pacis, check out these books on Amazon.com or from your library:
The Artists of the Ara Pacis: The Process of Hellenization in Roman Relief Sculpture, by Diane Atnally Conlin (1997). Specialized, but an award-winning study of the Greek influence on the carving of the frieze - concluding that Roman carvers did the actual work.
Roman Art: Romulus to Constantine, by Nancy and Andrew Ramage (4th Edition 2004). The standard textbook.