Energy or heat moves easily through some kinds of matter. Metals are good conductors because energy travels quickly through them , heating all the atoms along the way. Insulators are objects that don't conduct well. Plastic and wood are good insulators. Energy does not move well through them by conduction.
Heat stays inside the object.
For example, a hot object surrounded by plastic will have a difficult time heating up the plastic compared to if it were metal. This is why we need insulation in out houses. To keep the heat inside during the winter, we need something in the walls that makes it difficult for the heat to escape. The air is made up of molecules. These molecules bounce off each other all of the time.