The average clicks indicates the relationship between the number of all clicks and the number of unique clicks.

It therefore shows how frequently links were clicked, when at least one link was clicked. In other words, this is the average number of clicks per person (and this concerns only people who clicked on a link).

An example should make this clear:

A newsletter was sent to 10 recipients. It was opened by four of them. Of these four, one clicks on a link twice and another clicks on three different links. The average number of clicks is therefore 5/2 = 2.5.