ELI5: standard deviation

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.