If the mean of the sample standard deviations is equal to the population standard deviation, then the sample standard deviations target the population standard deviation and are called an unbiased estimator.
If the mean of the sample standard deviations is not equal to the population standard deviation, then the sample standard deviations target the population standard deviation and are called a biased estimator.
A biased estimator regularly underestimates or overestimates the parameter.
An unbiased sample statistics are good estimators
A biased sample statistics are not good estimators.
