Thermal conductivity refers to the intrinsic ability of a material to transfer or conduct heat. It is one of the three methods of heat transfer, the other two being convection and radiation.
The lower the thermal conductivity of a material, the better the thermal performance (i.e. the slower heat will move across a material).
Thermal conductivity values vary substantially between material and are highly dependent on the structure of each specific material.
Some materials will have different thermal conductivity values depending on the direction of heat travel; these are anisotropic materials.
In these cases, heat moves more easily in a certain direction due to how the structure is arranged.
Materials with a higher thermal conductivity are good conductors of thermal energy. 
