Through the analysis of the nature of temperature and heat, I found that temperature is not the sign of the average kinetic energy of molecular thermal motion, but related to the frequency corresponding to the peak intensity of thermal radiation, and the temperature is positively correlated with the main frequency of molecular thermal motion. Based on this, the physical phenomena such as constant temperature but absorbing or releasing a large amount of heat during the phase transition of most substances can be well explained. At the same time, the physical phenomena such as the inverse relationship between specific gravity and specific heat of most solid materials are predicted and explained. Heat is a measure of the degree of change of molecular kinetic energy in the process of molecular temperature convergence. On the basis of a new understanding of temperature and heat, this paper intends to reinterpret and understand entropy increase, heat death and the second law of thermodynamics.Energy is the most common term in physics, but there are still many disputes and misunderstandings about the nature of energy. In this paper, the essence of energy is discussed in depth, and its physical significance is expounded on this basis. At the same time, by expounding the essential meaning of temperature and heat, it is applied to explain the abnormal correlation between temperature and heat during the phase change of water and the abnormal temperature distribution in various layers outside the sun.On the basis of re-examining the differences between the two forms of Planck's blackbody radiation formula (frequency, wavelength and energy intensity respectively), the relationship between blackbody temperature and the frequency corresponding to the peak radiation intensity is re-deduced, and the temperature of the sun surface is calculated to be 9932.78℃ by using this relationship, instead of 5522.85℃ calculated by Wien's law. The reason of uneven surface temperature of artificial earth satellite is also discussed.