I am so excited.
Here it is:
Diffusivity: "the property of a substance indicative of the rate at which a thermal disturbance, as a rise in temperature, will be transmitted through the substance."
So basically, you have a substance of something. And then you change a thermal property like the temperature. Of course it takes time for the heat to travel through all that substance and to completely change the temperature of your substance. And the diffusivity tells one something about how fast the substance will change.