In the field of statistics, we use numbers to describe the world around us. Because of limitations in time, money, and resources determining the values associated with the entire population is almost impossible. So, instead of computing the values for the entire population we take a sample from the population and perform the statistical computations for sample and use those values to estimate what those values are for the entire population.
Statistics, which are used to describe a sample, and a parameter, which is used to describe the entire population, use different symbols to distinguish between them. For example:
Understanding these distinguishes is important to be able to communicate the statistical results correctly.