Heat is a form of energy. Each object contains matter which is made of molecules. These molecules are in a constant state of motion. This gives them kinetic energy. Also, there is a force of attraction between the molecules. This gives them potential energy.
Heat energy is the internal energy of these molecules. More the internal energy, the hotter the body. We can define heat as a form of energy that flows. The SI unit of heat is joule (J) and the CGS unit is er