Very similar to, and often confused with, Conductance as described above. The difference to be noted is that Conductance involves area while Conductivity involves length.
For any particular substance, its Thermal Conductivity is the measure of the quantity of (heat) energy which flows through a unit length, in unit time, when there is a unit temperature difference between the two ends of the length.
The SI units for this measure are watts/metre degree kelvin [W / m °K]See the explanation given above, under Conductance, as to why there is no mention of time in those units. Once again, there is in the older units.
Thermal Conductivity can be thought of indicating how quickly the heat will get to you. Imagine holding a bar and putting one end of it in a fire (like a poker). The end you were holding would warm up until (eventually) it became too hot to hold. How long this would take would depend upon the length of the bar and the material of which it was made. The table shows that silver (with a high value) would be quicker in getting the heat to your hand than one made of steel (with a lower value). A bar of concrete would be much slower still. There are other materials whose values are even lower, but it might be rather difficult (or impossible) to use those.
Approximate values in W / m °K of the Thermal Conductivities of some substances are:

THERMAL RESISTIVITY is the inverse of Conductivity.