Thermal Conductivity
Thermal Conductivity is how quickly heat moves through a material, like metal warming fast in your hand.
Think of it this way
A metal spoon and a wooden spoon both sit in a pot of hot soup. Heat moves up through the metal spoon fast, and the handle feels hot in seconds. The wooden spoon stays cool much longer because wood holds heat back. That gap in speed is thermal conductivity (how fast heat flows through a material).
Explaining thermal conductivity by grade level
Touch a metal spoon and a wooden spoon at the same time. The metal one feels cooler because heat leaves your hand faster through metal. Some metals move heat faster than others. Copper warms up the quickest of all.