Thermal conduction involves energy in the form of heat being transferred from a hot region to a cooler region through a material. At the hotter end the atoms, molecules, and electrons vibrate with more energy than they do at the cooler end. The atoms, molecules, and electrons generally don't flow from one place to the other - instead the energy flows through the material, passed along by the vibrations.
The rate at which heat is conducted along a bar of length L depends on the length, the cross-sectional area A, the temperature difference between the hot and cold ends, TH - TC, and the thermal conductivity k of the material.
The rate of energy transfer is power, so:
Metals generally have high thermal conductivities because of the free electrons that move around randomly. These are very efficient at transferring energy through the metal. Copper, for instance, has a thermal conductivity of 400 W/(m K), compared to 0.024 W/(m K) for foam insulation.
Insulating materials are rated in terms of their R values, which measures their resistance to conduction. The higher the R, the lower the conductivity. In terms of the thickness L: