To answer this question, the first thing we should know is what is hot? Hot water means that the water molecules (water is made of these molecules) are more likely to move and more kinematic than they are in the cold state. Let me give an example. Imagine we have two classrooms of kids. In classroom A, those kids are happy to move around in the room and do not want to stay in the same place while in classroom B most of kids are relatively quiet and do not want to move a lot. We can say classroom A is "hot", while classroom B is "cold". If we open a door between the two classroom and allow the kids to move around anywhere in the two classroom. Then we can say the kids are in a state that is between hot and cold.

To keep the water hot, you want the water molecules to stay excited. First thing you can do is to supply energy to those water molecules, such as using the electric power to heat the water up. Or correspondingly in the example above, to supply the candy bars and motivate the kids to move around. Second thing you can do is stop the water molecules from losing energy to outside. In this case, it is just keeping the door of the classroom shut, so that the active kids don't go outside. You know that a cup of hot water leaving in the air will become cold after a few hours. That is because the air is colder than the hot water, after the contact between the water and air, the heat from hot water will lose to the air. If you put the hot water in the specially designed cup, the water will still be warm after a few hours, because of the designs (thermally insulating or vacuum) to slow down the process of losing heat.