There are two common uses of the term "historiography."
In this case, the historiography of a topic is the sum total of the interpretations of a specific topic written by past and current historians. This allows historians and scholars to talk about "the state of the historiography" at a point in time, or to "add historiography" to a paper to make it more complete.
In this usage, a historiography or historiographical paper is an analysis of the interpretations of a specific topic written by past historians. Specifically, a historiography identifies influential thinkers and reveals the shape of the scholarly debate on a particular subject.
Alternately, a historiography can act as an introduction to a major research paper, in which you will go on to add your own analysis. As an introduction, a good historiography does the following:
"The historiography of the decision to use the atomic bomb on Hiroshima changed over the years as new research questioned the former consensus view that the decision to drop the atomic bomb was predicated on the necessity to save American lives."