Molecules in objects are constantly vibrating or moving inside objects. We cannot see that movement with our naked eye. When we heat the object this vibration and movement of molecules increases and temperature of the object also increases.
Thus, Heat is an energy that raises the temperature of a thing by causing the molecules in that thing to move faster.
Heat is not a matter. It doesn’t occupy space. It has no weight. Like light, sound and electricity, heat is a form of energy.
In short, Heat is the total kinetic energy of constituent particles of objects. SI Unit of Heat is joule. The unit calorie is also used.