Do you know what the (standard deviation) is ?

Theoretically,Standard deviation is the measure of deviance of observations from the mean of a given random variable 

For example :- A random variable has 100 observations with mean = 50 

Now if you take any one observation , for example 25 , it is deviated from the mean to a some extend. 

A histogram can graphically show the extend of deviation in a random variable. 

A random variable which is not deviated that much will follow a normal distributed. 

Normally distributed random variable is ideal for predictive modelling.