Lets look at what standard deviation is. It is a measure of variability from the mean. So lets say that you have a factory that produces jeans. The factor can make 200 jeans per hour. This means that the mean production is 200.
However. not every single machine operates in the same way - some machines might be old, some might be not tunes properly, and sometimes, the person who operates the machine is having a bad day. ...
An example of deriving standard deviation for a normal distribution