Thermal conductors and insulators
A good thermal conductor is a material that transfers thermal energy quickly.
All metals (e.g. iron, copper or gold) are thermal conductors. This is because they transfer thermal energy by free electron diffusion.
A thermal insulator is a material that conducts thermal energy very slowly.
Most non-metals like plastic or wood are thermal insulators. This is because insulators transfer thermal energy by atomic collisions only.
A perfect thermal insulator transfers no thermal energy at all.
A vacuum is a perfect insulator as there are no particles to conduct the heat.