What does the standard deviation of a data set describe?
The standard deviation of a data set describes how much do the data differ from their mean.
The standard deviation of a data set describes the difference between the data in the set and their mean. The simpliest interpretation could be: "The higher deviation, the more differences there are in the data set". It is rarely non-zero. In fact, as far as I know, the only possibility for a data set to have zero deviation is when it contains only the same numbers.
Let there be 2 companies A and B.
In company A the mean salary is $3,000 and the deviation $50.
In company B the mean salary is $2,800 and the deviation $300.
The interpretation of these values is that in company A the salary is between $2,950 and $3,050, while in B it is between $2,500 and $3,100.
The difference in deviation shows, that in A salaries are close to the mean, while in B the range of salaries is much bigger.