In the Prologue, Momaday says that “In one sense, then, the way to Rainy Mountain is preeminently the history of an idea, man’s idea of himself, and it has old and essential being in language.” What does the author mean?

Momaday’s The Way to Rainy Mountain traces the history of the Kiowa tribe, of which Momaday is a member. In the prologue of the book, Momaday spends the majority of the time explaining to the reader why he wanted to write this text and what he hopes to accomplish in doing so.
The above referenced quote comes in a passage in which Momaday discusses the purpose of telling the Kiowa migration story. The text will trace how the Kiowa tribe eventually settled in and around Rainy Mountain in Oklahoma, which is what the title of the text refers to and what this quote implies.
Momaday means that to explain how the Kiowa arrived at this location, he must explain what the Kiowa believed about themselves as a people. When he says it has essential “being in language,” he means that this idea—collective self-conception—is older than the written word but also that the Kiowa’a self-conception is present in the oral tradition that was passed down through the generations. As a result, the only way Momaday can accurately tell the Kiowa story is to examine the somewhat limited, fragmented oral traditions of his culture.