It acted as a crossroads for many different cultures, and ended up influencing each other in some way.
Cultural diffusion, I would think, is the exchange of knowledge and social practices among different cultures.
The Silk Road was a famous trade route crossing thousands of miles and allowing communication through trade and travel between the East and the West; hence, the cultural diffusion.
It means the spreading of the ideas and products of different cultures among each other. The Silk Road did this because traders from many cultures as far apart as Europe and China were constantly moving back and forth, bringing new ideas, new materials, new technologies to all the nations along the road.
Hopes this helps.
