The standard error of the mean is a measure of how much random sampling error is likely to be in a sample mean. That is, how far is the sample mean likely to be from the true population mean due to random sampling error?

All statistics have a standard error, but the standard error of the mean is the one most often calculated and virtually the only one ever reported as a descriptive statistic.

To be technically correct, the calculation above is of the estimated standard error of the mean, but the distinction is rarely made (or necessary) outside of mathematical statistics.