As you drive awai from the BTS, the radio signal becomes weaker. You would predict the strenght of the radio signal probably with help of the Okumura-Hata formula.
This is a decreasing curve in the chart. Now imagine, that you take measurements. Drive away from the BTS and the signal becomes weaker. When you compare, how the signal is decreasing in the theoretical curve and in the mesurements, you would see that the measured signal is some times lower and some times higher than the predicted one.
The differences can be expressed by the mathematical term “Standard Deviation”. Refere also to the mathematial model “Gaussian distribution”.