In a nutshell, historiography is the examination and analysis of existing scholarship on a historical topic. You look at various academic sources to see what other historians are saying about the topic and how they use primary sources, reasoning, and analysis to help understand it. Often you will find that there are differing schools of thought on the matter, and a good historiographical study will assess the various "sides" and examine their respective arguments and use of sources to see which seem better supported and reasoned.