"Conduction is the flow of heat through an unequally heated body from places of higher to places of lower temperature" - James Clerk Maxwell ^1
In general, Heat Transfer is a process where thermal energy is transferred due to a temperature difference.
The three modes of heat transfer are: Conduction, Convection, and Radiation.
Since conduction takes place from one material to another, it is easier for the elements to conduct their heat if they are tightly packed. Hence heavy materials like cooper have a very high thermal conductivity compared to lighter ones like polystyrene.
If you want to learn more about Conduction, please have a look into our SimWiki.
For more interesting and related articles as well as answers to questions, go to our SimScale SimWiki.
^1 : “Theory of heat” by J. Clerk Maxwell - 1871