Good question. Let's first understand why things are "hot". Everything is made of little balls called atoms, few of which clump together to form many small molecules. A thing feel hot because of these little balls vibrating, bouncing around - the faster they vibrate and bounce around, the hotter the thing is; slower the atoms the colder the thing is.

When fast moving atoms of the hotter thing comes in contact and bumps into slower atoms or molecules belonging to a colder thing, the hotter thing cools down, while the colder thing heats up. This is why a cup of hot coffee cools down: the fast coffee molecules bump into slow air molecules and become slower.

So, to keep things hot for a very long time, one must prevent the "slowing down" of these atoms and molecules. This can be done by isolating the hot thing from everything around it. If there are no slow molecules around to bump into, they will remain hot for a long time. This is the idea behind a "vacuum flask". Vacuum is literally nothing. No atoms, no molecules in it. In the flask, the hot liquid is separated/isolated from air by a thick space containing vacuum (or "nothing"). With nothing to bump into, the liquid remains hot for a long time.

Another process through which things cool is by emitting light (radiation). To avoid cooling through this process, one must surround the liquid by a mirror that reflects the emitted light back. Be on the lookout for an "Infra-red camera" in a science exhibition and trying imaging yourself with it. You will see how much light your body emits.

Keeping liquids hot is all about preventing heat from escaping. Heat flows through different materials at different speeds. Heat generally flows through things like metal very quickly and through things like wood and plastic very slowly. Interestingly, heat flows very slowly through still air. This is the basis of the designs of a lot of winter jackets, such as down jackets, which aim to trap pockets of air. If the heat from your body can only flow through the still air trapped in the jacket, the heat will flow slowly and you will be warmer. One famous example of a way people have come up with to keep liquids hot is the thermos flask. This consists of a bottle with two layers, with almost all of the air in-between the two layers sucked out. Because there is almost no material between the two layers of the bottle, heat can only flow very very slowly, and the liquid inside stays warm for much longer.

Are you asking about a thermos? Materials that trap air or, better yet, vacuum, are good insulators.

Materials that are good insulators keep liquids hot for a longer time. Examples of good insulators are silicon, various polymers that form gels and foams, and glass.