An Average Line works in the same way as a Constant Line. But unlike it, you don't enter any value. Instead, Tableau automatically calculates the average of the Measure. When you start to drag Average Line on the View, Tableau displays a menu where you can drop the option on Table, Pane, or Cell as shown in the following screenshot: 

The icons help you understand the difference between the three options:

• Table: Tableau draws a unique line that is the average of all the Marks
• Pane: Tableau draws as many lines as there are intersections between the Dimensions (in the preceding example, three lines)
• Cell: Tableau draws a line for each different Dimension combinations (so it's usually not very useful when using averages)

When you click on an Average Line, you can edit the default aggregation, average, to a median, or sum for example. If you click on Edit, Tableau opens a menu where you can customize the Average Line. Here's the screenshot of the menu:

In this menu, you can change the Scope (Table, Pane, or Cell), the value used for the average, the aggregation, the label, the format, and many other formatting options. If you want to change anything regarding your line, you can find it here.

The next option combines a Reference Line and Distribution Band.