Let’s say you are 5 years old and your father is 30. The average between you two is 35/2 =17.5.
\ Now let’s say your two cousins are 17 and 18. The average between them is also 17.5.
\ As you can see, the average alone doesn’t tell you much about the actual numbers. Enter standard deviation. Your cousins have a 0.5 standard deviation while you and your father have 12.5.
\ The standard deviation tells you how close are the values to the average. The lower the standard deviation, the less spread around are the values.