A thermos has an inner container made of glass. This container is really like one bottle inside of anouther bottle and sealed at the ends by melting the edges together. The air is removed out from the space between the two bottles to produce a vacuum, which is not a good conductor of heat and does a good job at slowing down the movement of heat. To slow down the other heat they coat the facing surfaces of the glass bottles with a silvery coating (like a mirror). This reflects the heat and helps slow down any losses that might get that way. They use a material like cork or rubber to make the bottle stopper and anything else that might touch the outside surface of the glass container. These materials are bad conductors of heat and slow down the heat loss. Whats left is what we can see on the outside which can be metal or plastic and is the covering for the glass bottles.
The idea here (with the thermos) is to slow down the movement of heat from one place to the other. So if you have hot stuff in the thermos or cold stuff in the thermos, the end result is the same. Keep heat from moving to where you don't want it to go for as long as possible.