I am not entirely sure I understand correctly what you mean by "essence". However, the idea of finding the standard deviation is to determine, as a general tendency, whether most data points are close to the average, or whether there is a large spread in the data. The standard deviation means, more or less, "How far is the typical data point from the average?"
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Standard deviation is a measure of the spread of data.
No, if the standard deviation is small the data is less dispersed.
Standard deviation is the variance from the mean of the data.
The smaller the standard deviation, the closer together the data is. A standard deviation of 0 tells you that every number is the same.
You cannot. If you are told the standard deviation of a variable there is no way to tell whether that was derived from grouped or ungrouped data.