W/mK stands for Watts per meter-Kelvin. It’s also known as ‘k Value’. The comparison of thermal conductivity can be measured by the ‘k’ value. The k value, or Thermal Conductivity, specifies the rate of heat transfer in any homogeneous material. If a material has a k value of 1, it means a 1m cube of material will transfer heat at a rate of 1 watt for every degree of temperature difference between opposite faces. The k value is expressed as 1 W/mK. The lower this value is, the less heat the material will transfer.
This can be very helpful in selecting the thermal materials you might need when attaching a heatsink to a component and is a specification filter to help narrow the options for Thermal pads.