The Sampling Distribution and Standard Error of the Mean
We will use an artificial population of five numbers, which are 1, 2, 3, 4, 5 (I choose not to use some hypothetical distribution of all the students in the USA or the like). If you want, assume that it is after Y2K and only 5 people survive - they are the population. We test them on some variable. One person says 1, another says 2, etc.

So our population is: 1, 2, 3, 4, 5. Note that the mean (m) for this population is 3 and its standard deviation is 1.414.