Any population is usually best described by its central tendency and variability measures. Well, these parameters should be used to best interpreting for information on a specific property under study.
Central tendency measures the value that mostly whole data/elements are centered around or grouped at. Which means, data values tend to be like/close to this value.
The most common methodology the describe the central tendency is the mean value, also called the expected value, or average value (usually for samples). There still more methods to indicate central tendency such as mode and median.
The average (or mean) of a sample of data is just the summation divided by the data count. Whereas, the mode is the most repeated/frequent value within our data. Also the median represent the middle ordered value when sorting the data group in ascending manner.
Example: the water (or juice) bottles production is a population where we can think of the bottle capacity as an important feature/property. When we look at whole 500 ml bottles, capacity values tend to be 500 ml (which is the mean or expected value). Notice that despite all bottles capacities should be close to 500 ml, they must not be exactly 500 ml. So, some can be 499.5 ml, 500.2...etc. In general, values tend to a specific center, which is the mean value.