The average verbal sat score for the entire class of entering freshmen is 530. however, if you select a sample of 20 freshmen and compute their average verbal sat score you probably will not get exactly 530. what statistical concept is used to explain the natural difference that exists between a sample mean and the corresponding population mean?

This difference is explained by the concept of statistical error. Statistical error simply means the difference between the 'true value' of some statistic (like the population mean) and the estimated value of some sample statistic (like a sample mean). Since no sample perfectly represents any population, there is always some statistical error involved when trying to use sample means to predict population means.