Thermal conductivity is the ability of a material to conduct heat. Metals are good at heat conduction. They are good conductors of heat. Gases are also good at moving heat. They are also good conductors of heat. Thermal resistivity is the opposite of thermal conductivity. This means to resist heat, or not conduct heat well.
Thermal conductivity is often represented by the Greek letter "kappa", . The units of thermal conductivity are watts per meter-kelvin. Watts are a measure of power, meters are a measure of length, and kelvins are a measure of temperature. From the units, we can see that thermal conductivity is a measure of how much power (in watts) moves through a distance (in meters) due to a temperature difference (in kelvin).
An example of thermal conductivity is when you put 2 spoons in boiling water. One spoon is steel and the other is silver. When you take the spoons out of the boiling water, the silver spoon is hotter than the steel spoon. The reason for this is that silver conducts heat better than steel.