about “molecular diffusivity”

When I simulate 2D model,I don’t know what does the “molecular diffusivity” mean? how can I difine the value? Can it possible equal to a value between  6600 x [10-9 m2s-1] and 18600 x [10-9 m2s-1]?
thanks

for example:
http://www.chromatography-online.org/topics/diffusivity.html