A grand narrative is an idea that is comprehensive in its incorporation of history and knowledge. Another word for a grand narrative is "metanarrative." The term "meta" indicates that it is essentially a story about a story: a description of a body of descriptions.
The term grand narrative was originally coined by Jean-Francois Lyotard. His 1979 work, "The Postmodern Condition: A Report on Knowledge," criticizes the legitimacy of grand narratives, and their claim to encompass the totality of knowledge on a particular subject.
Examples of grand narratives are philosophies such as democracy, Marxism and the Enlightenment. Lyotard's argument is that in a post-modern world, these philosophies are no longer valid. Instead, smaller, more specific narratives hold more water.