In literal terms, a metanarrative means a "big story". It represents, in short, an explanation for everything that happens in a society. In Sociology, the concept of a metanarrative is sometimes referred-to as a "high level theory" or, more-usually, a perspective / ideology.
Sociological perspectives such as Functionalism, Marxism, Interactionism and Feminism are all examples of what post-modernists call metanarratives, precisely because they attempt to account for all aspects of a society in terms of the perspective and the various theories it proposes (called "mid-range" or "mid-level theories" in technical terms. This type of theory will be used to explain one aspect of a particular sociological problem - for example, the mid-level theory of "cultural deprivation" has been used by Functionalist sociologists to explain differential educational achievement)
Outside of sociology / social science, various political and economic metanarratives could be noted. The concepts of "Capitalism", for example, or "Communism" and "Fascism" are examples of metanarratives, as are things like "Religion" (Roman Catholicism, Islam, Protestantism and so forth) and, probably most controversially, "Science" (and, in particular, the "Big Science" that argues, ultimately, all of the Laws governing the behaviour of matter in the universe can be discovered and, eventually, related to a single, unified, Law)
One argument frequently put-forward against the post-modernist critique of metanarratives is based on the observation of two facts:
Taylor ("Investigating Culture and Identity"), for example, notes various examples of metanarrative belief that fall into one - or sometimes both - of the above categories (Christian Fundamentalism in America and the Iranian (Muslim) Fundamentalist revolution, to name but two...).
However, this type of criticism rather misses the point of the argument. The argument, to reiterate, is not that "metanarratives" do not / cannot exist (since they plainly do and, it could be argued, their continued existence can be taken as evidence of post-modern social developments). Rather, the argument is that such metanarratives are, fundamentally (to coin a phrase) misconceived. That is, a belief in the power, vitality and validity of beliefs based around grand narratives are, so the post-modern argument goes, doomed to disappointment.
In many ways we can see some truth in this idea since all of the great "isms" we can identify in their metanarrative form have bloomed only to wither relatively quickly. If you think about the Iranian revolution, for example, after 20-odd years of a particular version of Islamic fundamentalism, changes are starting to appear- a move towards democratic forms of election, the easing of restrictions placed on male and female behaviour and the like.