The correct answer to this open question is the following.
According to the chart, who was giving what to whom?
According to the chart, the Han Dynasty was giving silk in catties and in bales to the Xiongnu Mongols, as part of the tribute paid by the Chinese to the Mongols that invaded them.
The Xiongnu Mongols used to raid the northern territories of China. They were known to be fierce and effective warriors that rode horses and dabble into the Chinese territory. That is why -or at least part of the reason- the Chinese emperor ordered the construction of the Chinese Great Wall.