Definition
Mean value of set of numbers is sum these numbers devided by amount.
Usage
Imagine you have 5 customers. Each paid you different amount of money: \1200, $1100, $1150, $900, $1500$.
You can get insight on how much money you make on average from each customer by calculating mean value: (\1200 + $1100 + $1150 + $900 + $1500) / 5 = $1170$
But some customers bring more money and some less? Yes, but they’ll balance each out. Some customers will pay you more then $1170 and some less.
\1200 + $1100 + $1150 + $900 + $1500 = 5 * $1170 = $5850$
The calculating mean value is basically answering the question: I made 5850.
Mean value allows you to ignore variance in your data and treat values as equal. But this can lead to problems in situations when variance cannot. In such situations it might be better to use Median
true man - population mean