This distinction is important and it is the reason that we need inferential statistics. We could randomly select a sample of 50 men and calculate their average height. Suppose we want to know the mean height of adult males in the U.S. We can use sampling to estimate the population mean (which we cannot know for certain). For large groups (say all adult males in the united states), finding this mean is impractical.
Population mean is the arithmetic mean of the whole population.
