La Mancha is the region of central Spain where the book begins, and the gentleman carries it in his name: "Don Quixote de la Mancha" is the name he chooses for himself, imitating the knights of the books, who add to their names that of their homeland.
Chapter I explains the choice. Amadis, says the narrator, was not content to be called plain Amadis, and added the name of his kingdom and homeland, Gaul; so the gentleman, as a good knight, wanted to add his homeland's name to his own and call himself Don Quixote de la Mancha, by which, in his view, he declared his lineage and homeland most vividly and honored them by taking their surname. The narrator, for his part, opens the book by saying that the gentleman lived "in a village of La Mancha, whose name I do not wish to recall," and never says which one.
The region enters the book wherever the book passes. El Toboso, Dulcinea's homeland, is, in the gentleman's words, "a village of La Mancha" (chapter XIII of Part One), and the Ruidera lagoons, which Sancho and his master see in La Mancha, serve as a point of comparison when the two see the sea for the first time, in Barcelona.
The narrator's silence finds its close in the last chapter of Part Two. Cide Hamete says he did not wish to pinpoint the gentleman's village, so that all the towns and villages of La Mancha might compete for the right to claim him, just as the seven cities of Greece competed for Homer. The name of the region is thus all the homeland the book gives the hero.