Insulator: A material that does not easily allow energy, such as heat, electricity, light, or sound to pass through it.
FYI: A material that does not transmit heat easily will not easily transmit other types of energy.
For example: Since wood does not easily transmit heat, wood is used for cooking utensils. You can hold these utensils and stir very hot food without the utensil getting hot and burning your hand.
Metals are not good insulators. If a material is not a good insulator, the other choice is that it is a good CONDUCTOR.
Conductors easily transmit energy. This means that energy is able to move through a conductor. Metals are good conductors.
For example: The metal spoon in the diagram is
laying on the metal handle of the metal cooking pot. It appears that the pot is sitting on top of camp fire.
Since metal is a good conductor of energy, the spoon gets hot because heat is transferred through the metal pot to its handle, and then to the metal spoon.