The qualification ``great'' (or ``small'') makes sense when accompanied by a quantity that serves as reference; else it's meaningless, because units are definitions of convention, not physics. So 10^4 K is a ``high'' temperature, if the unit used is the Kelvin-and a low temperature, if the unit used is the mega-Kelvin. The meaning of T_0 is contained in the meaning of the quantities that define it. Any temperature defines an energy scale, E=k_B T, where k_B = 1.38 x 10^(-23) J/K, is Boltzmann's constant; so a temperature of 10^4 K represents an energy scale of 1.38 x 10^(-19) J, which is slightly less than 1 eV. 10^5 K are about 10 eV  (which is slightly less than the ionization energy of hydrogen) and 10^9 K is 10^5 eV = 0.1 MeV. 

You should look at the Mott's theory. This is just a notation. It is inversely proportional to the density of states at the Fermi level and the cube of the localization length. Certainly, the assumptions of the theory are no longer valid long before such temperature is reached. The value is high because the electrons are very localized and have far to jump.    

Dear Stan,

Many thanks for your comment.

You're right, the energy corresponding to Mott’s temperature T_0 = 10^5 K is comparable with the ionization potential of the hydrogen atom. And the atomic hydrogen - almost ideal dielectric. However Mott’s theory of variable range hopping was created to explain the low-temperature conductivity of lightly doped semiconductors, where the energy barrier is less than 1 eV and temperatures are low (< 100 K). Especially confusing are the values T_0 = 10^9 K, obtained by other authors. Such huge barriers as deltaE = k_B * T_0 for the movement of electrons in semiconductors do not exist. The resulting energy is even greater than the Fermi energy of electrons in a metal. At the same time the voltage applied to the specimen in the process of measurement of hopping conductivity is only several Volts. Apparently RESONANT electron tunneling plays an important role in this mechanism. And here I do not completely understand the role of Mott’s temperature. Especially such a huge temperature as T_0 = 10^5 -:- 10^9 K.

Thank you again.