Heat energy is determined by how active the atoms or molecules are in a substance. The atoms and molecules in a hot substance move rapidly and seem excited while the molecules and atoms in a cool substance show less movement.
Heat energy also causes a substance to expand because the molecules need more space to move around in, and this is called thermal expansion. When a substance is cooled down, it will contract and be smaller than its original size. Linear expansion is the term use for rods or railway tracks where they will expand and become longer as they get warmer. Area expansion can be found in objects, such as engine pistons and the object becomes larger. Volume expansion can be seen in liquids as they become warmer.
Heat energy can transfer between objects. Conduction, convection and radiation are all terms use in heat transfer. Conduction is the action of heat transferring through solids, convection is the action of heat transferring through liquids and gasses, and radiation is heat transferred through visible and infrared light, such as that transferred by the sun.
Thermal equilibrium happens when a cool substance and a warm substance meet. The hot molecules loose energy to the cold molecules and they go back to having an energy level around the level they both started off at. Racecar drivers often leave their cars idling when cooling it down in order for all of the engine parts to reach thermal equilibrium.