The "Matter of Rome" is the study of the history and culture of ancient Rome.
This means learning about how the people who lived in Rome a long time ago built buildings, made art, fought wars, and lived their daily lives. We study their language (Latin), their laws, their system of government, and their religion.
All of this information helps us better understand how ancient Rome was important to the development of western culture, and how it has continued to influence the world even after it fell apart a long time ago.