Energy can be transferred usefully, stored or dissipated, but it can never be created or destroyed. This is the principle of conservation of energy. In a closed system there is no net change to the total energy. For example, a kettle transfers electrical energy to the thermal store of the water, and the total energy at the end equals the total at the start.
In every change some energy is dissipated. It spreads out into the surroundings and is stored in less useful ways, mostly as thermal energy. This is often described as being wasted. A motor lifting a load also warms its bearings and the air around it, so not all of the input energy ends up lifting the load.
Unwanted energy transfers can be reduced. Lubrication, such as oil on a bicycle chain, reduces friction between moving parts, so less energy is dissipated as heat. Thermal insulation, such as loft insulation in a house, reduces the rate of energy transfer out of a building.
Energy moves through a solid by conduction. The higher the thermal conductivity of a material, the higher the rate of energy transfer across it. Metals have high thermal conductivity and are used for saucepans. A building cools more slowly if its walls are thicker and are made from a material with a low thermal conductivity.