Energy stored inside a system by the particles (atoms and molecules that make up the system). This is called internal energy. Internal energy is the total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system
Some materialsrequire more energy to increase their temperature by 1°C than others. Eg you need 4200J to warm 1 kg of water by 1°C, but only 139J to warm 1 kg of mercury by 1°C.
Materials with high specific heat capacity need to gain lots of energy in their thermalenergy stores to heat up, but also release a lot of energy when they cooldown. They can store a lot of energy.