The mean average is a quick mathematical measure of a number of data points as a unit. It will tell you important information about a group of data in your business. It is almost a summary of all the data in your dataset.
This gives some important information about the length of widgets between line #1 and line #2. You would be justified in saying that the widgets from line #2 are longer than from line #1. Statisticians have a more complex way of coming to the same conclusion, but this is sufficient for simple applications. This does not mean that you couldn't find a widget from line #1 with the same length as a widget from line #2. Clearly the mean average is just a generalization for a given set of data.
The calculation is very simple. You simply add up all the values of your data points. Then you divide the total by the number of data points that you added together.
Most of these problems can be solved by also using the standard deviation as an add on measure of the data. If you click to What is Standard Deviation, you will find out what it is.
The mean average is a very convenient way of describing a lot of data. As long as you are confident that it is not skewed, you can use it by itself to as a convenient descriptor of the value of the data.