Ok, so historiography just means the study of how people have looked at and written about history over time. So, when we talk about the historiography of Japan, we mean thinking about how different people have approached writing about Japan's past.
Now, Japan has a REALLY long history - much longer than the US or Canada or other countries you might be familiar with - so people have been writing about it for thousands of years! But, different people have had different ideas about what parts of Japan's past are most important, how to interpret those events, and even what counts as "history" in the first place.
So, to study the historiography of Japan, we look at all of the different ways people have approached writing about Japan's past. In ancient times, people told stories and myths about Japan's founding, while later on, scholars wrote detailed accounts of Japan's emperors and important events in the country's history.
One interesting thing about the historiography of Japan is that it's been shaped a lot by politics, too. For example, during the time when Japan was ruled by shoguns (military commanders, kind of like emperors), many historians wrote about the power struggles between different shoguns and their followers. In the 20th century, after Japan became a democracy, there were debates about how to teach schoolchildren about Japan's role in World War II.
Overall, the historiography of Japan teaches us that history is always being written and re-written, and that people's ideas about the past are heavily influenced by the world around them.