The average of some part of a group (called a sample) is the usual way to estimate the average for the whole group. It is often too hard or too costly to measure the whole group. But if a different sample is measured, it will have an average that is a little bit different from the first sample. The standard error of the mean is a way to know how close the average of the sample is to the average of the whole group. It is a way of knowing how sure one can be about the average from the sample.