There are three ways of transferring heat: thermal radiation, thermal convection and thermal conduction.
People in the sun's rays, will feel warm, this is because the sun's heat hit us, this is called the radiation of heat.
The best way to prevent thermal radiation is to push it back. The best material to reflect heat is a mirror.
Put a cup of hot water on the table, because the water in the cup and the surrounding air flow, so that the temperature of the water gradually become the same as the ambient temperature, this is the convection of heat.
If you put a cover on the cup, you block the way of the convection. But the glass of water will still get cold, just over time. This is because the cup has the property of heat transfer, which is called heat conduction.