The internal energy or thermal energy of a system is the total kinetic energy of its molecules. At a molecular or atomic level, all objects are constantly in motion.
Temperature is an indirect measure of internal energy.
It is critical to remember that, although temperature is related to internal energy, they are not equivalent. Two bodies are in thermal equilibrium if they have the same temperature . If we place any two bodies in contact with each other, we observe that they eventually arrive at the same temperature. This is not true of the internal energies of the two systems for two reasons. First, internal energy depends on the amount of matter in a system.