This equation uses the Stefan-Boltzman law to compute the longwave emissions changes as a function of dTv (using the derivative form of Stefan-Boltzman law with respect to temperature), the epsilon_g is the emissivity, sigma is the Stefan-Boltzman constant, T_g,v is the temperature of ground/vegetation, dT_g,v is the temperature change for ground/vegetation
Hope this is helpful for you.