The difference between x̄ (x-bar) and μ (mu)

 In statistics, the symbol x̄ (x-bar) represents the sample mean or average of a set of data. It is calculated by adding up all of the values in the sample and dividing by the total number of values in the sample. The Greek letter mu (μ) represents the population mean or average of a larger group of data that the sample is drawn from. It is calculated in the same way as x-bar, but it represents the true mean of the entire population, rather than just the sample.

The difference between x̄ (x-bar) and μ (mu) is that x-bar represents the average of a sample of data, while mu represents the true average of the entire population from which the sample is drawn. Because samples are inherently imperfect and may not perfectly reflect the larger population, x-bar and mu may be different from each other.

Reference
Yakir, B. (2011). Introduction to statistical thinking (with R, without Calculus). The Hebrew University of Jerusalem, Department of Statistics.