Is there a way to define 26mV thermal voltage with simple words
Intuitively, it is the mean potential caused by the thermal motion of electrons. 

It is 

\$ \displaystyle V_T=\frac{kT}{q}\$ 

where T is the absolute temperature, k is the Boltzman constant and q is the charge on an electron. 



If you would like to think of the cloud of electrons as a gas it is also 

\$\displaystyle V_T=\frac{RT}{F}\$ 

where T is the absolute temperature, R is the gas constant (which relates energy to temperature) and F is the Faraday constant (charge per mole of electrons).