The correct answer to this open question is the following.
Unfortunately, you did not attach any text, article, context, or reference to know who the authors are and what they are specifically talking about.
However, trying to help you, we can comment on the following.
One of the most impressive things that some authors mention when they talk about Cahokia and led archaeologists to conclude that it was a major trading center, was the large mounds built in that location that could have served to store food or goods that were part of the things they traded.
Cahokia was an ancient Native American settlement located in the territory of what today is Illinois, approximately in 700AD. They were talented people capable of constructing those amazing and large mounds. Archeologists say that this place was the largest city north of México before the arrival of Christopher Columbus. They were known to be good traders that used the Mississippi River to navigate, transport people, and goods to other regions.
